Pin DefaultGatewayObservationConvention behavior at unit level

Adds focused unit assertions for the gateway client observation convention
beyond the metric name renamed in the previous commit:

- guard that getName() does not equal Spring Web's http.client.requests
- getContextualName span naming
- low-cardinality key set (http.method, http.status_code, and both
  spring.cloud.gateway.route.* keys)

Previously only covered indirectly by the integration-style
ObservedHttpHeadersFilterTests.

See gh-3153

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

Author: LivingLikeKrillin

spring-cloud-gateway-server-webflux/src/test/java/org/springframework/cloud/gateway/filter/headers/observation/DefaultGatewayObservationConventionTests.java

@@ -16,8 +16,16 @@
 
 package org.springframework.cloud.gateway.filter.headers.observation;
 
+import io.micrometer.common.KeyValue;
+import io.micrometer.common.KeyValues;
 import org.junit.jupiter.api.Test;
 
+import org.springframework.cloud.gateway.route.Route;
+import org.springframework.cloud.gateway.support.ServerWebExchangeUtils;
+import org.springframework.http.HttpHeaders;
+import org.springframework.mock.http.server.reactive.MockServerHttpRequest;
+import org.springframework.mock.web.server.MockServerWebExchange;
+
 import static org.assertj.core.api.Assertions.assertThat;
 
 class DefaultGatewayObservationConventionTests {
@@ -28,4 +36,37 @@ void getName_returnsSpringCloudGatewayPrefixedName() {
 			.isEqualTo("spring.cloud.gateway.http.client.requests");
 	}
 
+	@Test
+	void getName_doesNotCollideWithSpringWebClientMetric() {
+		assertThat(DefaultGatewayObservationConvention.INSTANCE.getName()).isNotEqualTo("http.client.requests");
+	}
+
+	@Test
+	void getContextualName_returnsHttpMethodPrefixedName() {
+		MockServerHttpRequest request = MockServerHttpRequest.get("http://localhost:8080/foo").build();
+		MockServerWebExchange exchange = MockServerWebExchange.from(request);
+		GatewayContext context = new GatewayContext(new HttpHeaders(), exchange.getRequest(), exchange);
+
+		assertThat(DefaultGatewayObservationConvention.INSTANCE.getContextualName(context)).isEqualTo("HTTP GET");
+	}
+
+	@Test
+	void getLowCardinalityKeyValues_pinsGatewayRouteKeys() {
+		MockServerHttpRequest request = MockServerHttpRequest.get("http://localhost:8080/foo").build();
+		Route route = Route.async()
+			.id("test-route")
+			.uri("http://localhost:8080/")
+			.order(1)
+			.predicate(exchange -> true)
+			.build();
+		MockServerWebExchange exchange = MockServerWebExchange.from(request);
+		exchange.getAttributes().put(ServerWebExchangeUtils.GATEWAY_ROUTE_ATTR, route);
+		GatewayContext context = new GatewayContext(new HttpHeaders(), exchange.getRequest(), exchange);
+
+		KeyValues keyValues = DefaultGatewayObservationConvention.INSTANCE.getLowCardinalityKeyValues(context);
+
+		assertThat(keyValues.stream().map(KeyValue::getKey)).containsExactlyInAnyOrder("http.method",
+				"http.status_code", "spring.cloud.gateway.route.id", "spring.cloud.gateway.route.uri");
+	}
+
 }