Reuben Pereira authored Dec 09, 2019 and Thong Kuah committed Dec 09, 2019

Create new service to do template variable replacement on the query
parameter of the Prometheus proxy API.

Adds a new endpoint which proxies calls to the grafana endpoint
which proxies calls to any grafana datasource configured to
support it. This will be used as a part of embedding prometheus
metrics in GFM via grafana.

- In prometheus proxy api, allow variables to be replaced. For example,
if 'up{env="%{ci_environment_slug}"}' is passed to the endpoint, it
becomes 'up{env="production"}' before being sent to prometheus.

- It is now being done in ProxyService class.

The api will proxy requests to the environment's prometheus server.
The Prometheus::ProxyService class can be reused when we add support for
group prometheus servers.

- It is now being done in ProxyService class.

The api will proxy requests to the environment's prometheus server.
The Prometheus::ProxyService class can be reused when we add support for
group prometheus servers.