Expose Prometheus metrics at /metrics

Exposes metrics:

* http_request_duration_seconds_bucket
* http_request_size_bytes_bucket
* http_response_size_bytes_bucket
* http_requests_total

with labels of action and controller to help identify popular
endpoints and their performance characteristics.

src/lib/Hydra.pm

@@ -16,7 +16,8 @@ use Catalyst qw/ConfigLoader
                 Session
                 Session::Store::FastMmap
                 Session::State::Cookie
-                Captcha/,
+                Captcha
+                PrometheusTiny/,
                 '-Log=warn,fatal,error';
 use CatalystX::RoleApplicator;
 use YAML qw(LoadFile);
@@ -47,6 +48,9 @@ __PACKAGE__->config(
             file($ENV{'HYDRA_LDAP_CONFIG'})
         ) : undef
     },
+    'Plugin::PrometheusTiny' => {
+        include_action_labels => 1,
+    },
     'Plugin::Static::Simple' => {
         send_etag => 1,
         expires => 3600