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.

t/Controller/metrics.t

@@ -0,0 +1,30 @@
+use feature 'unicode_strings';
+use strict;
+use Setup;
+use JSON qw(decode_json encode_json);
+
+my %ctx = test_init();
+
+require Hydra::Schema;
+require Hydra::Model::DB;
+require Hydra::Helper::Nix;
+use HTTP::Request::Common;
+
+use Test2::V0;
+require Catalyst::Test;
+Catalyst::Test->import('Hydra');
+
+my $db = Hydra::Model::DB->new;
+hydra_setup($db);
+
+request(GET '/');
+my $metrics = request(GET '/metrics');
+ok($metrics->is_success);
+
+like(
+    $metrics->content,
+    qr/http_requests_total\{action="index",code="200",controller="Hydra::Controller::Root",method="GET"\} 1/,
+    "Metrics are collected"
+);
+
+done_testing;