Add documentation pages for Caffeine and Guava cache instrumentation libraries

docs/content/instrumentation/caffeine.md

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+---
+title: Caffeine Cache
+weight: 1
+---
+
+The Caffeine instrumentation module, added in version 1.3.2, translates observability data
+provided by caffeine `Cache` objects into prometheus metrics.
+
+{{< tabs "uniqueid" >}}
+{{< tab "Gradle" >}}
+```
+implementation 'io.prometheus:prometheus-metrics-instrumentation-caffeine:1.3.2'
+```
+{{< /tab >}}
+{{< tab "Maven" >}}
+```xml
+<dependency>
+    <groupId>io.prometheus</groupId>
+    <artifactId>prometheus-metrics-instrumentation-caffeine</artifactId>
+    <version>1.3.2</version>
+</dependency>
+```
+{{< /tab >}}
+{{< /tabs >}}
+
+In order to collect metrics:
+
+ * A single `CacheMetricsCollector` instance must be registered with the registry;
+   * Multiple `CacheMetricsCollector` instances cannot be registered with the same registry;
+ * The `Cache` object must be instantiated with the `recordStats()` option, and then added to the
+   `CacheMetricsCollector` instance with a unique name, which will be used as the value of the
+   `cache` label on the exported metrics;
+   * If the `recordStats` option is not set, most metrics will only return zero values;
+
+```java
+var cache = Caffeine.newBuilder().recordStats().build();
+var cacheMetrics = CacheMetricsCollector.builder().build();
+PrometheusRegistry.defaultRegistry.register(cacheMetrics);
+cacheMetrics.addCache("mycache", cache);
+```
+
+{{< hint type=note >}}
+
+In version 1.3.5 and older of the caffeine instrumentation library, `CacheMetricsCollector.builder`
+does not exist, i.e. a constructor call `new CacheMetricsCollector()` is the only option.
+
+{{< /hint >}}
+
+All example metrics on this page will use the `mycache` label value.
+
+Generic Cache Metrics
+---------------------
+
+For all cache instances, the following metrics will be available:
+
+```
+# TYPE caffeine_cache_hit counter
+# HELP caffeine_cache_hit Cache hit totals
+caffeine_cache_hit_total{cache="mycache"} 10.0
+# TYPE caffeine_cache_miss counter
+# HELP caffeine_cache_miss Cache miss totals
+caffeine_cache_miss_total{cache="mycache"} 3.0
+# TYPE caffeine_cache_requests counter
+# HELP caffeine_cache_requests Cache request totals, hits + misses
+caffeine_cache_requests_total{cache="mycache"} 13.0
+# TYPE caffeine_cache_eviction counter
+# HELP caffeine_cache_eviction Cache eviction totals, doesn't include manually removed entries
+caffeine_cache_eviction_total{cache="mycache"} 1.0
+# TYPE caffeine_cache_estimated_size
+# HELP caffeine_cache_estimated_size Estimated cache size
+caffeine_cache_estimated_size{cache="mycache"} 5.0
+```
+
+Loading Cache Metrics
+---------------------
+
+If the cache is an instance of `LoadingCache`, i.e. it is built with a `loader` function that is
+managed by the cache library, then metrics for observing load time and load failures become
+available:
+
+```
+# TYPE caffeine_cache_load_failure counter
+# HELP caffeine_cache_load_failure Cache load failures
+caffeine_cache_load_failure_total{cache="mycache"} 10.0
+# TYPE caffeine_cache_loads counter
+# HELP caffeine_cache_loads Cache loads: both success and failures
+caffeine_cache_loads_total{cache="mycache"} 3.0
+# TYPE caffeine_cache_load_duration_seconds summary
+# HELP caffeine_cache_load_duration_seconds Cache load duration: both success and failures
+caffeine_cache_load_duration_seconds_count{cache="mycache"} 7.0
+caffeine_cache_load_duration_seconds_sum{cache="mycache"} 0.0034
+```
+
+Weighted Cache Metrics
+----------------------
+
+Two metrics exist for observability specifically of caches that define a `weigher`:
+
+```
+# TYPE caffeine_cache_eviction_weight counter
+# HELP caffeine_cache_eviction_weight Weight of evicted cache entries, doesn't include manually removed entries
+caffeine_cache_eviction_weight_total{cache="mycache"} 5.0
+# TYPE caffeine_cache_weighted_size gauge
+# HELP caffeine_cache_weighted_size Approximate accumulated weight of cache entries
+caffeine_cache_weighted_size{cache="mycache"} 30.0
+```
+
+{{< hint type=note >}}
+
+`caffeine_cache_weighted_size` is available only if the cache instance defines a `maximumWeight`.
+
+{{< /hint >}}
+
+Up to version 1.3.5 and older, the weighted metrics had a different behavior:
+
+ * `caffeine_cache_weighted_size` was not implemented;
+ * `caffeine_cache_eviction_weight` was exposed as a `gauge`;
+
+It is possible to restore the behavior of version 1.3.5 and older, by either:
+
+ * Using the deprecated `new CacheMetricsCollector()` constructor;
+ * Using the flags provided on the `CacheMetricsCollector.Builder` object to opt-out of each of the
+   elements of the post-1.3.5 behavior:
+   * `builder.collectWeightedSize(false)` will disable collection of `caffeine_cache_weighted_size`;
+   * `builder.collectEvictionWeightAsCounter(false)` will expose `caffeine_cache_eviction_weight` as
+     a `gauge` metric;

docs/content/instrumentation/guava.md

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+---
+title: Guava Cache
+weight: 1
+---
+
+The Guava instrumentation module, added in version 1.3.2, translates observability data
+provided by Guava `Cache` objects into prometheus metrics.
+
+{{< tabs "uniqueid" >}}
+{{< tab "Gradle" >}}
+```
+implementation 'io.prometheus:prometheus-metrics-instrumentation-guava:1.3.2'
+```
+{{< /tab >}}
+{{< tab "Maven" >}}
+```xml
+<dependency>
+    <groupId>io.prometheus</groupId>
+    <artifactId>prometheus-metrics-instrumentation-guava</artifactId>
+    <version>1.3.2</version>
+</dependency>
+```
+{{< /tab >}}
+{{< /tabs >}}
+
+In order to collect metrics:
+
+* A single `CacheMetricsCollector` instance must be registered with the registry;
+  * Multiple `CacheMetricsCollector` instances cannot be registered with the same registry;
+* The `Cache` object must be instantiated with the `recordStats()` option, and then added to the
+  `CacheMetricsCollector` instance with a unique name, which will be used as the value of the
+  `cache` label on the exported metrics;
+  * If the `recordStats` option is not set, most metrics will only return zero values;
+
+```java
+var cache = CacheBuilder.newBuilder().recordStats().build();
+var cacheMetrics = new CacheMetricsCollector();
+PrometheusRegistry.defaultRegistry.register(cacheMetrics);
+cacheMetrics.addCache("mycache", cache);
+```
+
+All example metrics on this page will use the `mycache` label value.
+
+Generic Cache Metrics
+---------------------
+
+For all cache instances, the following metrics will be available:
+
+```
+# TYPE guava_cache_hit counter
+# HELP guava_cache_hit Cache hit totals
+guava_cache_hit_total{cache="mycache"} 10.0
+# TYPE guava_cache_miss counter
+# HELP guava_cache_miss Cache miss totals
+guava_cache_miss_total{cache="mycache"} 3.0
+# TYPE guava_cache_requests counter
+# HELP guava_cache_requests Cache request totals
+guava_cache_requests_total{cache="mycache"} 13.0
+# TYPE guava_cache_eviction counter
+# HELP guava_cache_eviction Cache eviction totals, doesn't include manually removed entries
+guava_cache_eviction_total{cache="mycache"} 1.0
+# TYPE guava_cache_size
+# HELP guava_cache_size Cache size
+guava_cache_size{cache="mycache"} 5.0
+```
+
+Loading Cache Metrics
+---------------------
+
+If the cache is an instance of `LoadingCache`, i.e. it is built with a `loader` function that is
+managed by the cache library, then metrics for observing load time and load failures become
+available:
+
+```
+# TYPE guava_cache_load_failure counter
+# HELP guava_cache_load_failure Cache load failures
+guava_cache_load_failure_total{cache="mycache"} 10.0
+# TYPE guava_cache_loads counter
+# HELP guava_cache_loads Cache loads: both success and failures
+guava_cache_loads_total{cache="mycache"} 3.0
+# TYPE guava_cache_load_duration_seconds summary
+# HELP guava_cache_load_duration_seconds Cache load duration: both success and failures
+guava_cache_load_duration_seconds_count{cache="mycache"} 7.0
+guava_cache_load_duration_seconds_sum{cache="mycache"} 0.0034
+```