[prometheus-exporter] add server and port env vars

[naseemkullah]

Is your feature request related to a problem? Please describe.

While in production I tend to use the default port across the board, it is nice when testing frameworks can choose a port at random, as such i usually set any server ports to 0 when testing such that they grab any available port.

To avoid 9464 already in use issues during tests I'd like an easy way to set the prometheux exporter's server port.

Describe the solution you'd like

Implement https://github.com/open-telemetry/opentelemetry-specification/blob/master/specification/sdk-environment-variables.md#prometheus-exporter

[vmarchaud]

Done with #1857 so i'm closing this one