Queriers: Fix external search config when using RBAC with Google Cloud Run

[modulitos]

What this PR does:

I implemented a feature enabling RBAC on Google Cloud Run endpoints, which was merged last week: #2593

But there is a bug with the configuration, as described below. This PR fixes that bug.

Repro:

Run Tempo with the following config:

querier:
  search:
    external_backend: google_cloud_run
    google_cloud_run:
      external_endpoints: ["https://my-cloud-run-endpoint.run.app"]

Open the Grafana Tempo explore panel, and search for traces.
We'll see traces in our results, but they should be coming from our external Cloud Run endpoint, not an internal block search.

Expected:

Block queries are sent to the external Cloud Run endpoint.

Actual:

An internal search is performed instead of an external search. This is because the querier.search.external_endpoints list is unset, so it defaults to an empty list, and is caught by this conditional:

tempo/modules/querier/querier.go

Lines 716 to 718 in 41dfbc4

	if len(q.cfg.Search.ExternalEndpoints) == 0 {
	return q.internalSearchBlock(ctx, req)
	}

We can work around it by defining a non-empty querier.search.external_endpoints list:

querier:
  search:
    external_endpoints: ["dummy-value"]
    external_backend: google_cloud_run
    google_cloud_run:
      external_endpoints: ["https://my-cloud-run-endpoint.run.app"]

but clearly that's not desirable. This PR fixes the conditional and adds a unit test to cover this code path.

Checklist

• Tests updated
• Documentation added
  • No documentation changes required
• CHANGELOG.md updated - the order of entries should be [CHANGE], [FEATURE], [ENHANCEMENT], [BUGFIX]
  • Since this is a bug on a pre-release feature, I figured it's not worth updating the changelog. Happy to update it though if requested.

[mapno]

LGTM.

cc/ @joe-elliott, bugfix to include in v2.2

[joe-elliott]

i think this can be a cleaner check in the future, but i'm fine with it for now.