Basic Prometheus metrics for API requests

[psrok1]

Your checklist for this pull request

• I've read the contributing guideline.
• I've tested my changes by building and running the project, and testing changed functionality (if applicable)
• I've added automated tests for my change (if applicable, optional)
• I've updated documentation to reflect my change (if applicable)

What is the new behaviour?

/api/varz endpoint that exposes basic Prometheus metrics:

# HELP api_requests API request metrics
# TYPE api_requests gauge
api_requests{endpoint="api.serverdocsresource",method="GET",status_code="200",user="admin"} 2.0
api_requests{endpoint="api.fileresource",method="GET",status_code="200",user="admin"} 3.0
api_requests{endpoint="api.configstatsresource",method="GET",status_code="200",user="admin"} 1.0
api_requests{endpoint="api.configresource",method="GET",status_code="200",user="admin"} 1.0
api_requests{endpoint="api.metricsresource",method="GET",status_code="200",user="admin"} 1.0
api_requests{endpoint="api.loginresource",method="POST",status_code="200",user="172.19.0.7"} 1.0
api_requests{endpoint="api.quickqueryresource",method="GET",status_code="200",user="admin"} 4.0
api_requests{endpoint="api.userlistresource",method="GET",status_code="200",user="admin"} 4.0
api_requests{endpoint="api.remotelistresource",method="GET",status_code="200",user="admin"} 2.0
api_requests{endpoint="api.serverinforesource",method="GET",status_code="200",user="admin"} 1.0
api_requests{endpoint="api.serveradmininforesource",method="GET",status_code="200",user="admin"} 1.0

Endpoint requires authentication (API key) with access_prometheus_metrics capability.

TODO: Documentation