fix: change headless service to gRPC and expose 9094 TCP

[locmai]

What this PR does

Follow-up the work of @AlexandreRoux from #435 and related to #420 to run the helm-docs for the README.md

This PR corrects the alertmanager headless service with exposing gRPC port instead of HTTP.

I tested the Helm chart and I can't reproduce the EOF issue.

[nschad]

we should inform the user about this breaking chance in CHANGELOG.md

[locmai]

I have updated the CHANGELOG.md to make this clearer. Or do you think we should add a better indication that this may break if users were using the http-metrics port for their own usage? Similar to the old PR we have here #169

And by breaking - do you mean we should make it to v3 (too much I think?) or a line in the CHANGELOG should be good enough?

[nschad]

Hmmm. I don't have a strong opinion about it. Technically it should be a major release because it will break if somebody depends on it (somehow).

And to be honest so #169 should have been.

CC: @kd7lxl Do you have an opinion?

[kd7lxl]

I would just not touch the http-metrics port, so that there is no breaking change. Adding gRPC has nothing to do with the http-metrics port.

[locmai]

That sounds good. Let me update that.

[locmai]

Done!

[kd7lxl]

These should be ENHANCEMENT.