Loki: Add feature flag to enable dataplane-compliant metric frames

[gabor]

this PR adds a feature-flag that makes Loki's responses to metric queries compliant with the dataplane-spec's time series multi and numeric multi formats.

(similar PR happened to Prometheus in #62694)

this means the following changes:

• the Frame.Name field is not used anymore
• for "vector" data (numbers for a single timestamp), the Frame.Meta.Type field changes from timeseries-multi to numeric-multi.

we do not expect many backward-compatibility issues with this change, but for now it is opt-in thing controlled by the feature-flag.

we are running the snapshot-tests twice, both for the flag being enabled and also for the flag being disabled

when reading the diff, the real changes are in the tsdb/loki part, the rest is adding a feature-flag, and adding more snapshot-tests (because now we need to cover both flag-enabled and flag-disabled cases).

if anyone has any idea how to do this in an easier way, i'm listening 👍 .

how to test:

1. enable the feature-flag lokiMetricDataplane
2. go to explore, run a metric LogQL query, make sure the graph you see looks ok
3. repeat the same in a dashboard
4. now disable the feature-flag lokiMetricDataplane
5. repeat the tests from [2] and [3]

[github-actions]

Backend code coverage report for PR #66017

Plugin	Main	PR	Difference
loki	58.4%	58.4%	0%

[github-actions]

Frontend code coverage report for PR #66017
No changes

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[kylebrandt]

If "empty" means "no data" in Grafana terms here, it can be returned as a single frame with no fields. This way metadata can still be passed. In particular, the executed query string (if using macros, can help to see what happened to result in no data if unexpected). https://github.com/grafana/grafana-plugin-sdk-go/blob/main/data/contract_docs/contract.md#no-data-and-empty

[gabor]

thanks for the tip. i'll check what to modify to achieve this 👍

[gabor]

oh, i see prom adds an empty-frame if converter/prom.go returns zero frames:

grafana/pkg/tsdb/prometheus/querydata/response.go

Lines 34 to 37 in 5df9e64

	// Add frame to attach metadata
	if len(r.Frames) == 0 && !q.ExemplarQuery {
	r.Frames = append(r.Frames, data.NewFrame(""))
	}

i'll do the same for Loki too (in a separate PR, not to complicate things here)