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>

INSTALL

@@ -49,9 +49,15 @@ read the instructions below.
 
     # pacman -S python-mysql-connector python-pygit2 python-srcinfo python-sqlalchemy \
                 python-bleach python-markdown python-alembic python-jinja \
-                python-itsdangerous python-authlib python-httpx hypercorn
+                python-itsdangerous python-authlib python-httpx hypercorn \
+                composer
     # python3 setup.py install
 
+4a) Install `composer` dependencies while inside of aurweb's root:
+
+    $ cd /path/to/aurweb
+    /path/to/aurweb $ composer require promphp/prometheus_client_php
+
 5) Create a new MySQL database and a user and import the aurweb SQL schema:
 
     $ python -m aurweb.initdb