Create new observation context for WebClient retries

Prior to this commit, the `DefaultWebClient` observability
instrumentation would create the observation context before the reactive
pipeline is fully materialized. In case of errors and retries (with the
`retry(long)` operator), the observation context would be reused for
separate observations, which is incorrect.

This commit ensures that a new observation context is created for each
subscription.

Fixes gh-34671

spring-webflux/src/test/java/org/springframework/web/reactive/function/client/WebClientObservationTests.java

@@ -69,6 +69,7 @@ class WebClientObservationTests {
 		when(mockResponse.statusCode()).thenReturn(HttpStatus.OK);
 		when(mockResponse.headers()).thenReturn(new MockClientHeaders());
 		when(mockResponse.bodyToMono(Void.class)).thenReturn(Mono.empty());
+		when(mockResponse.bodyToMono(String.class)).thenReturn(Mono.error(IllegalStateException::new), Mono.just("Hello"));
 		when(mockResponse.bodyToFlux(String.class)).thenReturn(Flux.just("first", "second"));
 		when(mockResponse.releaseBody()).thenReturn(Mono.empty());
 		when(this.exchangeFunction.exchange(this.request.capture())).thenReturn(Mono.just(mockResponse));
@@ -141,6 +142,16 @@ class WebClientObservationTests {
 				.hasLowCardinalityKeyValue("status", "200");
 	}
 
+	@Test
+	void recordsSingleObservationForRetries() {
+		StepVerifier.create(this.builder.build().get().uri("/path").retrieve().bodyToMono(String.class).retry(1))
+				.expectNextCount(1)
+				.expectComplete()
+				.verify(Duration.ofSeconds(2));
+		assertThatHttpObservation().hasLowCardinalityKeyValue("outcome", "SUCCESS")
+				.hasLowCardinalityKeyValue("status", "200");
+	}
+
 	@Test
 	void setsCurrentObservationInReactorContext() {
 		ExchangeFilterFunction assertionFilter = (request, chain) -> chain.exchange(request).contextWrite(context -> {