Create new http.TargetServer shared across all HTTP Promtail Targets

[thepalbi]

What this PR does / why we need it:

This PR creates a wrapper around WeaveWorks HTTP server, to be able to re-use some common configuration all Promtail targets that expose and HTTP server do.

From all the targets: gcp logs push, heroku and lokipush expose and HTTP server, based on WeaveWorks. They all have some repeated logic that:

• applies some defaults to the config
• configures a metrics namespace
• toggles instrumentation and some other stuff

By extracting a common TargetServer, we can share all that configuration procedure, delegating to each target itself configuring just the HTTP handlers..

Which issue(s) this PR fixes:
Fixes #

Special notes for your reviewer:
lokipush was not pulled in the new wrapped sever, since it handles some configurations differently. Leaving as a follow up.

Checklist

• Reviewed the CONTRIBUTING.md guide (required)
• Documentation added
• Tests updated
• CHANGELOG.md updated
• Changes that require user attention or interaction to upgrade are documented in docs/sources/upgrading/_index.md

[thepalbi]

Closing in favour of grafana/agent#3581