linked metrics-endpoint (#132)

Co-authored-by: Michael Hoennig <michael@hoennig.de>
Reviewed-on: https://dev.hostsharing.net/hostsharing/hs.hsadmin.ng/pulls/132
Reviewed-by: Timotheus Pokorra <timotheus.pokorra@hostsharing.net>

src/main/java/net/hostsharing/hsadminng/config/CustomActuatorEndpoint.java

@@ -0,0 +1,44 @@
+package net.hostsharing.hsadminng.config;
+
+import lombok.Getter;
+import org.springframework.boot.actuate.endpoint.annotation.Endpoint;
+import org.springframework.boot.actuate.endpoint.annotation.ReadOperation;
+import org.springframework.stereotype.Component;
+import org.springframework.web.client.RestTemplate;
+import org.springframework.web.servlet.support.ServletUriComponentsBuilder;
+
+import java.util.List;
+
+@Component
+@Endpoint(id="metric-links")
+// BLOG: implement a custom Spring Actuator endpoint to view _clickable_ Spring Actuator (Micrometer) Metrics endpoints
+// HOWTO: implement a custom Spring Actuator endpoint
+public class CustomActuatorEndpoint {
+
+    private final RestTemplate restTemplate = new RestTemplate();
+
+    @ReadOperation
+    public String getMetricsLinks() {
+        final String baseUrl = ServletUriComponentsBuilder.fromCurrentContextPath().build().toUriString();
+        final var metricsEndpoint = baseUrl + "/actuator/metrics";
+
+        final var response = restTemplate.getForObject(metricsEndpoint, ActuatorMetricsEndpointResource.class);
+
+        if (response == null || response.getNames() == null) {
+            throw new IllegalStateException("no metrics available");
+        }
+        return generateJsonLinksToMetricEndpoints(response, metricsEndpoint);
+    }
+
+    private static String generateJsonLinksToMetricEndpoints(final ActuatorMetricsEndpointResource response, final String metricsEndpoint) {
+        final var links = response.getNames().stream()
+                .map(name -> "\"" + name + "\": \"" + metricsEndpoint + "/" + name + "\"")
+                .toList();
+        return "{\n" + String.join(",\n", links) + "\n}";
+    }
+
+    @Getter
+    private static class ActuatorMetricsEndpointResource {
+        private List<String> names;
+    }
+}

src/main/resources/application.yml

@@ -8,7 +8,8 @@ management:
     endpoints:
         web:
             exposure:
-                include: info, health, metrics
+                # HOWTO: view _clickable_ Spring Actuator (Micrometer) Metrics endpoints: http://localhost:8081/actuator/metric-links
+                include: info, health, metrics, metric-links
     observations:
         annotations:
             enabled: true