feat(prometheus.receive_http): Accept Pushgateway format (#1)

[meysam81]

Brief description of Pull Request

This PR will add support to receive Pushgateway-compatible metrics through a separate :9999 port (configurable) which allows Alloy to replace the current single-functionality of Prometheus Pushgateway.

Pull Request Details

I am currently using Alloy as the sole ingestor of metrics, logs, and traces on all my infrastructures and I needed to replace the Pushgateway with what's already a feature-complete product, that is Alloy.

This PR is going to serve myself first, and perhaps a bunch of other folks waiting for it in its respective GH issue.

I considered having it as an opt-in/beta feature but it is small enough to be rolled out on its own, since it's not a breaking functionality to the prometheus.receive_http at the moment.

Issue(s) fixed by this Pull Request

fixes #3493

Notes to the Reviewer

I ran the example end-to-end locally with the setup present in the current PR under example/pushgateway.

PR Checklist

• Documentation added
• Tests updated
• Config converters updated
• This pull request was substantially generated with AI assistance (see the GenAI policy)

Expand to see the updated rendered doc

[cla-assistant]

All committers have signed the CLA.