test: GCM tests for agent mode

[bwplotka]

Since we plan to experiment with the agent mode on GMP collection, let's ensure we have e2e tests against this mode:

prometheus/google/internal/promqle2etest/gcm_test.go

Line 69 in 85abb8b

func setupBackends(t testing.TB) (promqle2e.PrometheusBackend, PrometheusForkGCMBackend, LocalExportGCMBackend) {