[exporters/prometheus] Prometheus exporter mTLS implementation

[gfierka]

Is your feature request related to a problem? Please describe.
Many opentelemetry exporters support TLS, but that's not the case with prometheus exporter. Implementing TLS would boost security.

Describe the solution you'd like
From an end user perspective, implementing a solution similar to existing ones, i.e. Jaeger exporter

tls:
  key_file: file.key
  cert_file: file.cert

[neelayu]

#10858 will resolve this issue.

[neelayu]

#12921

[dmitryax]

Thanks for the contribution @neelayu