Do not instrument "hello"

When MongoObservationCommandListener is enabled according to documentation, there is a race in metrics registration:

> The meter (MeterId{name='spring.data.mongodb.command.active', tags=[tag(db.name=test),tag(db.operation=hello),tag(db.system=mongodb),tag(net.peer.name=localhost),tag(net.peer.port=27017),tag(net.transport=IP.TCP),tag(spring.data.mongodb.cluster_id=6840b93dabcd29b2f0526362)]}) registration has failed: ...

Prometheus does not allow register metric with different tags. The problem is that healthcheck calls Mongo command "hello" which does not refer to any collection. Consequently, tag "db.mongodb.collection" is absent for "hello" but present in regular commands.

Event if regular commands always wins the race (and metrics always has tag "collection"), the race must be removed.

My proposal is skip "hello" instrumentation, like "admin" commands

Signed-off-by: michaldo <[email protected]>

Author: michaldo

spring-data-mongodb/src/main/java/org/springframework/data/mongodb/observability/MongoObservationCommandListener.java

@@ -105,6 +105,10 @@ public void commandStarted(CommandStartedEvent event) {
 			return; // don't instrument commands like "endSessions"
 		}
 
+		if ("hello".equals(event.getCommandName())) {
+			return; // don't instrument healthcheck
+		}
+
 		RequestContext requestContext = event.getRequestContext();
 
 		if (requestContext == null) {

spring-data-mongodb/src/test/java/org/springframework/data/mongodb/observability/MongoObservationCommandListenerTests.java

@@ -98,6 +98,16 @@ void commandStartedShouldNotInstrumentWhenNoParentSampleInRequestContext() {
 		assertThat(meterRegistry).hasMeterWithName("spring.data.mongodb.command.active");
 	}
 
+	@Test
+	void commandStartedShouldNotInstrumentWhenHello() {
+
+		// when
+		listener.commandStarted(new CommandStartedEvent(new MapRequestContext(), 0, 0, null, "some name", "hello", null));
+
+		// then
+		assertThat(meterRegistry).hasNoMetrics();
+	}
+
 	@Test
 	void successfullyCompletedCommandShouldCreateTimerWhenParentSampleInRequestContext() {