Implement metrics endpoint in metrics lib (#42)

* Implement metrics endpoint in metrics lib

* Add tests for metrics endpoint

* Update METRICS.md

* Add tests for metrics endpoint

docs/METRICS.md

@@ -55,3 +55,35 @@ Kotlet.routing {
     // your routes
 }
 ```
+
+## Metrics scrape endpoint
+
+> [!IMPORTANT]
+> For using scrape metrics endpoint you must add to your dependencies
+> [prometheus-metrics-exporter-servlet-jakarta](https://mvnrepository.com/artifact/io.prometheus/prometheus-metrics-exporter-servlet-jakarta)
+> library.
+
+
+To expose metrics for Prometheus, you can use the `installMetricsScrape` method. Add it to your kotlet router like this:
+
+```kotlin
+val appRouting = Kotlet.routing {
+    installMetrics(kotletMetrics) // Now all requests of this routing will be measured
+    get("/hello") { call ->
+        call.respondText("Hello, World!")
+    }
+}
+
+val auxRouting = Kotlet.routing {
+    installMetricsScrape {
+        path = "/metrics"
+    }
+}
+
+// Combine routings
+Kotlet.servlet(
+    routings = listOf(appRouting, auxRouting)
+)
+```
+
+After it metrics will be available at `/metrics` endpoint in OpenMetrics format.

metrics/build.gradle.kts

@@ -6,12 +6,14 @@ dependencies {
     implementation(project(":core"))
     compileOnly(libs.jakarta.api)
     compileOnly(libs.prometheus.metrics.core)
+    compileOnly(libs.prometheus.metrics.exporter.servlet.jakarta)
 
     testImplementation(project(":mocks"))
     testImplementation(libs.kotlin.test)
     testImplementation(libs.mockk)
     testImplementation(libs.jakarta.api)
     testImplementation(libs.prometheus.metrics.core)
+    testImplementation(libs.prometheus.metrics.exporter.servlet.jakarta)
 }
 
 tasks.test {

metrics/src/main/kotlin/kotlet/metrics/MetricsScrape.kt

@@ -0,0 +1,57 @@
+package kotlet.metrics
+
+import io.prometheus.metrics.config.PrometheusProperties
+import io.prometheus.metrics.model.registry.PrometheusRegistry
+import kotlet.Routing
+import kotlet.prometheus.PrometheusScrapeEndpointHandler
+
+/**
+ * Install a scrape endpoint for Prometheus metrics.
+ */
+fun Routing.installMetricsScrape(configure: MetricsScrapeConfigBuilder.() -> Unit = {}) {
+    val builder = MetricsScrapeConfigBuilder()
+    builder.configure()
+    val config = builder.build()
+
+    get(config.path, PrometheusScrapeEndpointHandler(config.config, config.registry))
+}
+
+/**
+ * Configuration for the scrape endpoint.
+ */
+internal data class MetricsScrapeConfig(
+    val path: String,
+    val registry: PrometheusRegistry,
+    val config: PrometheusProperties
+)
+
+/**
+ * Builder for [MetricsScrapeConfig].
+ */
+class MetricsScrapeConfigBuilder internal constructor() {
+    /**
+     * Path to the scrape endpoint
+     * Default: /metrics
+     */
+    var path: String = "/metrics"
+
+    /**
+     * Prometheus registry to scrape metrics from
+     * Default: [PrometheusRegistry.defaultRegistry]
+     */
+    var registry: PrometheusRegistry = PrometheusRegistry.defaultRegistry
+
+    /**
+     * Prometheus configuration
+     * Default: [PrometheusProperties.get]
+     */
+    var config: PrometheusProperties = PrometheusProperties.get()
+
+    internal fun build(): MetricsScrapeConfig {
+        return MetricsScrapeConfig(
+            path = path,
+            registry = registry,
+            config = config,
+        )
+    }
+}

metrics/src/main/kotlin/kotlet/prometheus/PrometheusScrapeEndpointHandler.kt

@@ -0,0 +1,22 @@
+package kotlet.prometheus
+
+import io.prometheus.metrics.config.PrometheusProperties
+import io.prometheus.metrics.exporter.common.PrometheusScrapeHandler
+import io.prometheus.metrics.exporter.servlet.jakarta.HttpExchangeAdapter
+import io.prometheus.metrics.model.registry.PrometheusRegistry
+import kotlet.Handler
+import kotlet.HttpCall
+
+/**
+ * Handler for Prometheus scrape endpoint.
+ */
+internal class PrometheusScrapeEndpointHandler(
+    config: PrometheusProperties,
+    registry: PrometheusRegistry
+) : Handler {
+    private val scrapeHandler = PrometheusScrapeHandler(config, registry)
+
+    override fun invoke(call: HttpCall) {
+        scrapeHandler.handleRequest(HttpExchangeAdapter(call.rawRequest, call.rawResponse))
+    }
+}

metrics/src/test/kotlin/kotlet/metrics/MetricsScrapeUnitTest.kt

@@ -0,0 +1,63 @@
+package kotlet.metrics
+
+import io.prometheus.metrics.model.registry.PrometheusRegistry
+import kotlet.HttpMethod
+import kotlet.Kotlet
+import kotlet.mocks.Mocks
+import kotlet.prometheus.PrometheusMetricsCollector
+import java.time.Clock
+import java.time.Instant
+import java.time.ZoneId
+import kotlin.test.Test
+import kotlin.test.assertEquals
+
+class MetricsScrapeUnitTest {
+    @Test
+    fun testEndpoint() {
+        val clock = Clock.fixed(Instant.now(), ZoneId.of("UTC"))
+        val collector = PrometheusMetricsCollector(PrometheusRegistry.defaultRegistry, clock)
+
+        val route = Kotlet.routing {
+            installMetrics(collector)
+
+            get("/", {})
+
+            installMetricsScrape {
+                path = "/metrics"
+            }
+        }
+
+        val servlet = Kotlet.servlet(listOf(route))
+
+        val getCall = Mocks.httpCall(
+            method = HttpMethod.GET,
+            routePath = "/"
+        )
+        servlet.service(getCall.rawRequest, getCall.rawResponse)
+
+        val metricsCall = Mocks.httpCall(
+            method = HttpMethod.GET,
+            routePath = "/metrics"
+        )
+        servlet.service(metricsCall.rawRequest, metricsCall.rawResponse)
+
+        val body = metricsCall.responseData.toString(Charsets.UTF_8)
+        val expected = """
+            # HELP kotlet_http_requests_total Total number of HTTP requests
+            # TYPE kotlet_http_requests_total counter
+            kotlet_http_requests_total{method="GET",path="/",status="200"} 1.0
+            # HELP kotlet_http_requests_duration_seconds Duration of HTTP requests in seconds
+            # TYPE kotlet_http_requests_duration_seconds summary
+            kotlet_http_requests_duration_seconds{method="GET",path="/",status="200",quantile="0.5"} 0.0
+            kotlet_http_requests_duration_seconds{method="GET",path="/",status="200",quantile="0.9"} 0.0
+            kotlet_http_requests_duration_seconds{method="GET",path="/",status="200",quantile="0.95"} 0.0
+            kotlet_http_requests_duration_seconds{method="GET",path="/",status="200",quantile="0.99"} 0.0
+            kotlet_http_requests_duration_seconds_count{method="GET",path="/",status="200"} 1
+            kotlet_http_requests_duration_seconds_sum{method="GET",path="/",status="200"} 0.0
+
+            """.trimIndent()
+
+        assertEquals(expected, body)
+    }
+
+}

mocks/src/main/kotlin/kotlet/mocks/http/MockHttpCall.kt

@@ -37,8 +37,11 @@ class MockHttpCall(
     val responseData: ByteArray
         get() = responseStream.toByteArray()
 
+    val responseHeaders: MutableMap<String, String> = mutableMapOf()
+
     init {
         rawRequest = createHttpRequestMock(
+            path = routePath,
             methodName = httpMethod.name,
             async = async,
             headers = headers,
@@ -48,6 +51,9 @@ class MockHttpCall(
         val responseHeaders = mutableMapOf<String, String>()
         rawResponse = mockk {
             every { outputStream } returns ByteArrayServletOutputStream(responseStream)
+            every { setHeader(any(), any()) } answers {
+                responseHeaders[this.firstArg()] = this.secondArg()
+            }
 
             // contentTypeField is a private field, so we need to use a setter to set it
             every { contentType = any() } answers { contentTypeField = this.firstArg() }
@@ -72,6 +78,7 @@ class MockHttpCall(
 }
 
 private fun createHttpRequestMock(
+    path: String,
     methodName: String,
     async: Boolean,
     headers: Map<String, String>,
@@ -80,6 +87,8 @@ private fun createHttpRequestMock(
     val attributes = mutableMapOf<String, Any>()
 
     return mockk {
+        every { requestURI } returns path
+        every { queryString } returns ""
         every { inputStream } returns ByteArrayServletInputStream(requestData)
         every { getHeader(any()) } answers {
             headers[this.firstArg()]