Feat add prometheus auth

[holzgeist]

Hi there,

This PR adds support for basic auth on the prometheus server. We realized that it's open to the public when deploying via kubernetes and helm, as useHostnetwork is true to avoid UDP port range issues.

Any comments?

Thanks

[CLAassistant]

All committers have signed the CLA.

[davidzhao]

this is a good idea. the main feedback is it'd be great if we could consolidate prometheus options under a single object. I'm thinking something like:

prometheus:
  port: xxx
  username: xxx
  password: xxx

we'll make this backwards compatible and mark with a deprecation tag in the config. perhaps it's something we'll fully deprecate in 2.0

[holzgeist]

Thanks for your feedback, I updated the PR accordingly.
I'm not sure how to make it backwards compatible and deprecate it though. Is this something you add on top?