Add %{{TIME_NOW}} pattern for sprintf

docs/static/event-data.asciidoc

@@ -94,6 +94,23 @@ These formats are not directly interchangeable, and we advise you to begin using
 
 NOTE: A Logstash timestamp represents an instant on the UTC-timeline, so using sprintf formatters will produce results that may not align with your machine-local timezone.
 
+You can generate a fresh timestamp by using `%{{TIME_NOW}}` syntax instead of relying on the value in `@timestamp`.
+This is particularly useful when you need to estimate the time span of each plugin.
+
+[source,js]
+----------------------------------
+input {
+  heartbeat {
+    add_field => { "heartbeat_time" => "%{{TIME_NOW}}" }
+  }
+}
+filter {
+  mutate {
+    add_field => { "mutate_time" => "%{{TIME_NOW}}" }
+  }
+}
+----------------------------------
+
 [discrete]
 [[conditionals]]
 ==== Conditionals

logstash-core/src/main/java/org/logstash/StringInterpolation.java

@@ -32,6 +32,7 @@
 
 public final class StringInterpolation {
 
+    private static final String TIME_NOW = "TIME_NOW";
     private static final ThreadLocal<StringBuilder> STRING_BUILDER =
         new ThreadLocal<StringBuilder>() {
             @Override
@@ -83,13 +84,20 @@ public static String evaluate(final Event event, final String template) throws J
                 // JAVA-style @timestamp formatter:
                 // - `%{{yyyy-MM-dd}}` -> `2021-08-11`
                 // - `%{{YYYY-'W'ww}}` -> `2021-W32`
+                // A special pattern to generate a fresh current time
+                // - `%{{TIME_NOW}}` -> `2025-01-16T16:57:12.488955Z`
                 close = close + 1; // consume extra closing squiggle
-                final Timestamp t =  event.getTimestamp();
-                if (t != null) {
-                    final String javaTimeFormatPattern = template.substring(open+3, close-1);
-                    final java.time.format.DateTimeFormatter javaDateTimeFormatter = DateTimeFormatter.ofPattern(javaTimeFormatPattern).withZone(ZoneOffset.UTC);
-                    final String formattedTimestamp = javaDateTimeFormatter.format(t.toInstant());
-                    builder.append(formattedTimestamp);
+                final String pattern = template.substring(open+3, close-1);
+                if (pattern.equals(TIME_NOW)) {
+                    final Timestamp now = new Timestamp();
+                    builder.append(now);
+                } else {
+                    final Timestamp t = event.getTimestamp();
+                    if (t != null) {
+                        final DateTimeFormatter javaDateTimeFormatter = DateTimeFormatter.ofPattern(pattern).withZone(ZoneOffset.UTC);
+                        final String formattedTimestamp = javaDateTimeFormatter.format(t.toInstant());
+                        builder.append(formattedTimestamp);
+                    }
                 }
             } else if (template.charAt(open + 2) == '+') {
                 // JODA-style @timestamp formatter:

logstash-core/src/test/java/org/logstash/StringInterpolationTest.java

@@ -30,7 +30,9 @@
 import java.util.HashMap;
 import java.util.Map;
 
-import static org.junit.Assert.*;
+import static org.junit.Assert.assertEquals;
+import static org.junit.Assert.assertThrows;
+import static org.junit.Assert.assertTrue;
 
 
 public class StringInterpolationTest extends RubyTestBase {
@@ -149,6 +151,24 @@ public void TestValueIsHash() throws IOException {
         assertEquals("{\"k1\":\"v\"}", StringInterpolation.evaluate(event, path));
     }
 
+    @Test
+    public void TestTimeNow() throws IOException {
+        Event event = getTestEvent();
+        String pattern = "%{{TIME_NOW}}";
+        Timestamp before = new Timestamp();
+        Timestamp result = new Timestamp(StringInterpolation.evaluate(event, pattern));
+        Timestamp after = new Timestamp();
+        assertTrue(before.compareTo(result) < 0);
+        assertTrue(after.compareTo(result) > 0);
+    }
+
+    @Test
+    public void TestBadTimeNow() throws IOException {
+        Event event = getTestEvent();
+        String pattern = "%{{BAD_TIME_NOW}}";
+        assertThrows(IllegalArgumentException.class, () -> StringInterpolation.evaluate(event, pattern));
+    }
+
     public Event getTestEvent() {
         Map<String, Object> data = new HashMap<>();
         Map<String, String> inner = new HashMap<>();