Include K8 configuration bean for stackdriver monitoring in metrics starter

[dzou]

When deploying an app to kubernetes, in order to get JVM application metrics with proper labels (i.e. get RAM usage by containerName or podName) you must add the following bean:

    @Bean
    public static StackdriverMeterRegistry stackdriver() {

        return StackdriverMeterRegistry.builder(new StackdriverConfig() {
            @Override
            public String projectId() {
                return MetadataConfig.getProjectId();
            }

            @Override
            public String get(String key) {
                return null;
            }
            @Override
            public String resourceType() {
                return "k8s_container";
            }

            @Override
            public Map<String, String> resourceLabels() {
                Map<String, String> map = new HashMap<>();
                String podName = System.getenv("HOSTNAME");
                String containerName = podName.substring(0,
                    podName.indexOf("-"));
                map.put("location", MetadataConfig.getZone());
                map.put("container_name", containerName);
                map.put("pod_name", podName);
                map.put("cluster_name", MetadataConfig.getClusterName());
                map.put("namespace_name", System.getenv("NAMESPACE"));
                return map;
            }
        }).build();
    }

See this for reference:
https://github.com/GoogleCloudPlatform/bank-of-anthos/pull/289/files#diff-ea214d77e7f8cce9af9624c294899ab3

I wonder if we can somehow incorporate this into the metrics starter in a clean way rather than have users provide this configuration. Note that System.getenv("HOSTNAME") is only set in linux environment.

[elefeint]

Seems like a good idea. I am surprised Project ID is not autopopulated with our starter, though -- seems like it should, since our project autodetermination includes GKE.