fix: only allow aggregated metric queries from logs drilldown app

[trevorwhitney]

What this PR does / why we need it:

Aggregated metrics are stored as regular log streams in Loki, but are only used by the Logs Drilldown app. Requests from this app already identify themselves by setting the X-Query-Tags: Source=grafana-lokiexplore-app header. This change validates aggregated metric queries to look for this header, and return an empty iterator if not found. This will obfuscate the aggregated metric queries, which will prevent the confusion that has resulted in multiple issues/support requests around where these streams come from.

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

[salvacorts]

LGTM!