feat(runner/prometheus): PROMETHEUS_AUTH, URL query string, retention fallback

[mayankpande88]

Restore three prometheus config knobs the legacy agent honored:

• PROMETHEUS_AUTH: a static Authorization header applied to every
  Prometheus request (direct client + UI proxy path). Overlaid via Set
  so it takes precedence over any Authorization in PROMETHEUS_HEADERS.
• PROMETHEUS_URL_QUERY_STRING: appended to the query string of every
  Prometheus request (direct client + proxy), preserving existing params.
• PROMETHEUS_RETENTION_TIME: fallback retention value reported to the UI
  when /status/flags is unavailable (VictoriaMetrics vmsingle 404s it);
  also read VM's -retentionPeriod flag key when present.

Co-Authored-By: Claude Opus 4.8 (1M context) noreply@anthropic.com
Claude-Session: https://claude.ai/code/session_01L2HAXQH1gEtX7h4sUJsi9j

[gemini-code-assist]

Code Review

This pull request introduces support for several new Prometheus configuration variables, including PROMETHEUS_AUTH for static authorization, PROMETHEUS_URL_QUERY_STRING to append query parameters to Prometheus requests, and PROMETHEUS_RETENTION_TIME as a fallback retention value. It updates the agent runner, Prometheus client, and Grafana proxy to handle these configurations, and extends the retention probe to support VictoriaMetrics. Feedback on the changes points out discrepancies in the documentation and code comments regarding the delimiter for PROMETHEUS_HEADERS (which is parsed as comma-separated, not semicolon-separated), and suggests using net/url instead of manual string manipulation to safely append query parameters.

[gemini-code-assist]

The documentation states that PROMETHEUS_HEADERS is semicolon-separated, but the underlying parser config.ParseHeaders in runner/pkg/config/config.go splits headers by commas (,):

for _, part := range strings.Split(s, ",")

If users configure this variable using semicolons, it will fail to parse correctly (e.g., the second header will be treated as part of the first header's value). Please revert this to 'Comma-separated' or update the parser to support semicolons.

[gemini-code-assist]

This comment describes PROMETHEUS_HEADERS as semicolon-separated (Header: value; Header: value), but config.ParseHeaders actually parses them using commas (,) as delimiters. Please update the comment to reflect the actual comma-separated format to avoid confusion.

Suggested change

	// "Header: value; Header: value" string) — applied to every
	// "Header: value, Header: value" string) — applied to every

[gemini-code-assist]

Using manual string manipulation to append query parameters can be fragile (e.g., if the URL already ends with ? or &, or contains fragments). Since net/url is already imported in this file, we can leverage url.Parse to safely and robustly append the query string.

func appendQueryString(rawurl, extra string) string {
	extra = strings.TrimSpace(extra)
	extra = strings.TrimPrefix(extra, "?")
	if extra == "" {
		return rawurl
	}
	u, err := url.Parse(rawurl)
	if err != nil {
		sep := "?"
		if strings.Contains(rawurl, "?") {
			sep = "&"
		}
		return rawurl + sep + extra
	}
	if u.RawQuery == "" {
		u.RawQuery = extra
	} else {
		u.RawQuery += "&" + extra
	}
	return u.String()
}