fix: sanitize User-Agent to valid UTF-8 before using as Prometheus label value

[Segflow]

What this PR does / why we need it:

Sanitizes the User-Agent HTTP header to valid UTF-8 before passing it to prometheus.CounterVec.WithLabelValues() in the push path. Invalid UTF-8 bytes in the User-Agent (e.g. \xe5\xe5\xe5...) cause WithLabelValues to panic, crashing the distributor.

The fix uses strings.ToValidUTF8(userAgent, "") to strip invalid UTF-8 sequences before the value is used as a Prometheus label.

Which issue(s) this PR fixes:
Fixes distributor panic on push requests with invalid UTF-8 User-Agent headers.

Special notes for your reviewer:
The panic was observed in production with a User-Agent containing raw \xe5 bytes. The regression test uses a recent timestamp to ensure the distributorLagByUserAgent code path is exercised.

Checklist

• Reviewed the CONTRIBUTING.md guide (required)
• Documentation added
• Tests updated
• Title matches the required conventional commits format, see here
• 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.