Document rationale for gateway client observation metric name

Explains on DefaultGatewayObservationConvention#getName() why the
metric is prefixed with 'spring.cloud.gateway.' so a future reader
does not revert the rename without understanding the Prometheus
collision it avoids.

See gh-3153

Signed-off-by: JOOYOUNG JUNG <143606756+LivingLikeKrillin@users.noreply.github.com>

Author: LivingLikeKrillin

spring-cloud-gateway-server-webflux/src/main/java/org/springframework/cloud/gateway/filter/headers/observation/DefaultGatewayObservationConvention.java

@@ -70,6 +70,15 @@ public KeyValues getHighCardinalityKeyValues(GatewayContext context) {
 		return KeyValues.of(URI.withValue(context.getRequest().getURI().toString()));
 	}
 
+	/**
+	 * The {@code spring.cloud.gateway.} prefix avoids a metric name collision with Spring
+	 * Web's {@code DefaultClientRequestObservationConvention}, which uses
+	 * {@code http.client.requests} for {@code WebClient} requests with a different
+	 * low-cardinality label set. Without it, Prometheus refuses to register one of the
+	 * two meters and a metric series is dropped.
+	 * @see <a href=
+	 * "https://github.com/spring-cloud/spring-cloud-gateway/issues/3153">gh-3153</a>
+	 */
 	@Override
 	public String getName() {
 		return "spring.cloud.gateway.http.client.requests";