feat: adds User-Agent to distributor push logging

[aarogoss]

When capturing statistics regarding the most recent push request, extract the User-Agent header value from the request headers. If this value is included, strip off the trailing '/.*' content and include the agent value in the log line.

What this PR does / why we need it:

Which issue(s) this PR fixes:
Adds User-Agent to the distributor logging so we can track agent types along with lag metrics.

Special notes for your reviewer:

Checklist

• Reviewed the CONTRIBUTING.md guide (required)
• Documentation added
• Tests updated
• Title matches the required conventional commits format, see here
  • Note that Promtail is considered to be feature complete, and future development for logs collection will be in Grafana Alloy. As such, feat PRs are unlikely to be accepted unless a case can be made for the feature actually being a bug fix to existing behavior.
• Changes that require user attention or interaction to upgrade are documented in docs/sources/setup/upgrade/_index.md
• If the change is deprecating or removing a configuration option, update the deprecated-config.yaml and deleted-config.yaml files respectively in the tools/deprecated-config-checker directory. Example PR