feat(anubis): add /healthz route to metrics server

Also add health check test for Docker Compose and update documentation
for health checking Anubis with Docker Compose.

Signed-off-by: Xe Iaso <me@xeiaso.net>

internal/health.go

@@ -0,0 +1,25 @@
+package internal
+
+import (
+	"context"
+
+	"google.golang.org/grpc/health"
+	healthv1 "google.golang.org/grpc/health/grpc_health_v1"
+)
+
+var HealthSrv = health.NewServer()
+
+func SetHealth(svc string, status healthv1.HealthCheckResponse_ServingStatus) {
+	HealthSrv.SetServingStatus(svc, status)
+}
+
+func GetHealth(svc string) (healthv1.HealthCheckResponse_ServingStatus, bool) {
+	st, err := HealthSrv.Check(context.Background(), &healthv1.HealthCheckRequest{
+		Service: svc,
+	})
+	if err != nil {
+		return healthv1.HealthCheckResponse_UNKNOWN, false
+	}
+
+	return st.GetStatus(), true
+}