feat(PHP): add aurweb Prometheus metrics

Along with this initial requests metric implementation,
we also now serve the `/metrics` route, which grabs request
metrics out of cache and renders them properly for Prometheus.

**NOTE** Metrics are only enabled when the aurweb system admin
has enabled caching by configuring `options.cache` correctly
in `$AUR_CONFIG`. Otherwise, an error is logged about no cache
being configured.

New dependencies have been added which require the use of
`composer`. See `INSTALL` for the dependency section in regards
to composer dependencies and how to install them properly for
aurweb.

Metrics are in the following forms:

    aurweb_http_requests_count(method="GET",route="/some_route")
    aurweb_api_requests_count(method="GET",route="/rpc",type="search")

This should allow us to search through the requests for specific routes
and queries.

Signed-off-by: Kevin Morris <kevr@0cost.org>

web/html/metrics.php

@@ -0,0 +1,16 @@
+<?php
+/**
+ * Provide some variable Prometheus metrics. A new requests.route type
+ * gets created for each request made and we keep a count using our
+ * existing Memcached or APC configurable cache, with route
+ * = {request_uri}?{query_string}.
+ *
+ * TL;DR -- The 'requests' counter is used to give variable requests
+ * based on their request_uris and query_strings.
+ **/
+include_once('metricfuncs.inc.php');
+
+// Render metrics based on options.cache storage.
+render_metrics();
+
+?>