Add custom metric for rate limit remaining count

Now that we have Micrometer set up, we can create an additional,
custom metric for our application. In this case, we'd like to
track the remaining calls to the rate-limited API we're using.

dashboard/src/main/java/io/spring/sample/dashboard/stats/ReverseLookupClient.java

@@ -1,18 +1,23 @@
 package io.spring.sample.dashboard.stats;
 
+import java.util.concurrent.atomic.AtomicInteger;
+
+import io.micrometer.core.instrument.MeterRegistry;
 import io.spring.sample.dashboard.stats.support.ReverseLookupDescriptor;
 import reactor.core.publisher.Mono;
 
 import org.springframework.stereotype.Component;
+import org.springframework.util.StringUtils;
+import org.springframework.web.reactive.function.client.ExchangeFilterFunction;
 import org.springframework.web.reactive.function.client.WebClient;
 
 @Component
 public class ReverseLookupClient {
 
 	private final WebClient client;
 
-	public ReverseLookupClient(WebClient.Builder builder) {
-		this.client = builder.build();
+	public ReverseLookupClient(WebClient.Builder builder, MeterRegistry registry) {
+		this.client = builder.filter(rateLimitRemainingMetric(registry)).build();
 	}
 
 	public Mono<ReverseLookupDescriptor> freeReverseLookup(String ip) {
@@ -24,4 +29,17 @@ public Mono<ReverseLookupDescriptor> payingReverseLookup(String ip) {
 		return this.client.get().uri("http://localhost:8081/reverse-lookup/costly/{ip}", ip)
 				.retrieve().bodyToMono(ReverseLookupDescriptor.class);
 	}
+
+	private ExchangeFilterFunction rateLimitRemainingMetric(MeterRegistry registry) {
+		AtomicInteger rateLimitRemaining = registry
+				.gauge("reverselookup.ratelimit.remaining", new AtomicInteger(0));
+		return (request, next) -> next.exchange(request)
+				.doOnNext(response -> {
+					String remaining = response.headers().asHttpHeaders()
+							.getFirst("X-RateLimit-Remaining");
+					if (StringUtils.hasText(remaining)) {
+						rateLimitRemaining.set(Integer.valueOf(remaining));
+					}
+				});
+	}
 }