feat(SourceMetrics): knowledge graph single kg annotations POC

[bohandley]

✨ Description

Related issue(s):
#1037

Companion PR in knowledge graph plugin here https://github.com/grafana/asserts-app-plugin/pull/2949

POC to add single annotations (not multilane which is still being tested) to SoureMetrics component which is consumed by the RCA workbench in the knowledge graph.

We can show the annotations one of two ways:

1. Only in the metric graph (preferred, cleaner)

2. In all metric scene panels (is there any benefit to seeing them in this way? time range, comparison?)

📖 Summary of the changes

The asserts app passes entity context as primitives (kgDatasourceUid, entityType, entityName, entityScope) to the SourceMetrics exposed component. SourceMetrics builds a KgEntityConfig (plain object) and passes it into newMetricsTrail(). From there it flows down the constructor chain:

DataTrail → MetricScene → MetricGraphScene

MetricGraphScene calls buildKgAnnotationsLayer(kgEntityConfig) in its constructor, producing a fresh SceneDataLayerSet containing an AnnotationsDataLayer. This is set as $data on topView -- the SceneFlexLayout holding only the main chart. When the panel's SceneQueryRunner activates, Grafana Scenes traverses ancestors via sceneGraph.getDataLayers(), finds the layer, and runs it. The layer queries the KG datasource at /api-server/v1/search/assertions and the results render as annotation bands.

Breakdown panels can be unaffected because they live under MetricGraphScene.selectedTab, a separate subtree that doesn't pass through topView.

Files changed

MetricScene/kgAnnotations.ts -- new file containing KgEntityConfig, KgEntityScope types and buildKgAnnotationsLayer()
MetricScene/MetricGraphScene.tsx -- accepts kgEntityConfig, builds and sets the annotation layer as $data on topView
MetricScene/MetricScene.tsx -- threads kgEntityConfig through to MetricGraphScene
AppDataTrail/DataTrail.tsx -- stores kgEntityConfig in state, passes it to MetricScene on metric selection
exposedComponents/SourceMetrics/SourceMetrics.tsx -- accepts entity props, builds KgEntityConfig, passes to trail

🧪 How to test?

Run the asserts app with the changes here
Confirm that the annotations work in the source metrics component used in the RCA workbench

1. Click Frontend entity
2. Click the insight with ErrorRatioBreach
3. Click "metrics"

4. See the annotations in the Metrics Drilldown Metric Scene panel