Annotations/Alerting: Add Loki historian store stub (#78363)

* Add Loki historian store stub

* Add composite store

* Use composite store if Loki historian enabled

* Split store interface into read/write

* Make composite + historian stores read only

* Use variadic constructor for composite

* Modify Loki store enable logic

* Use dskit.concurrency.ForEachJob for parallelism

pkg/services/annotations/annotationsimpl/composite_store.go

@@ -0,0 +1,67 @@
+package annotationsimpl
+
+import (
+	"context"
+	"sort"
+
+	"github.com/grafana/dskit/concurrency"
+	"github.com/grafana/grafana/pkg/services/annotations"
+	"github.com/grafana/grafana/pkg/services/annotations/accesscontrol"
+)
+
+// CompositeStore is a read store that combines two or more read stores, and queries all stores in parallel.
+type CompositeStore struct {
+	readers []readStore
+}
+
+func NewCompositeStore(readers ...readStore) *CompositeStore {
+	return &CompositeStore{
+		readers: readers,
+	}
+}
+
+// Get returns annotations from all stores, and combines the results.
+func (c *CompositeStore) Get(ctx context.Context, query *annotations.ItemQuery, accessResources *accesscontrol.AccessResources) ([]*annotations.ItemDTO, error) {
+	itemCh := make(chan []*annotations.ItemDTO, len(c.readers))
+
+	err := concurrency.ForEachJob(ctx, len(c.readers), len(c.readers), func(ctx context.Context, i int) error {
+		items, err := c.readers[i].Get(ctx, query, accessResources)
+		itemCh <- items
+		return err
+	})
+	if err != nil {
+		return make([]*annotations.ItemDTO, 0), err
+	}
+
+	close(itemCh)
+	res := make([]*annotations.ItemDTO, 0)
+	for items := range itemCh {
+		res = append(res, items...)
+	}
+	sort.Sort(annotations.SortedItems(res))
+
+	return res, nil
+}
+
+// GetTags returns tags from all stores, and combines the results.
+func (c *CompositeStore) GetTags(ctx context.Context, query *annotations.TagsQuery) (annotations.FindTagsResult, error) {
+	resCh := make(chan annotations.FindTagsResult, len(c.readers))
+
+	err := concurrency.ForEachJob(ctx, len(c.readers), len(c.readers), func(ctx context.Context, i int) error {
+		res, err := c.readers[i].GetTags(ctx, query)
+		resCh <- res
+		return err
+	})
+	if err != nil {
+		return annotations.FindTagsResult{}, err
+	}
+
+	close(resCh)
+	res := make([]*annotations.TagsDTO, 0)
+	for r := range resCh {
+		res = append(res, r.Tags...)
+	}
+	sort.Sort(annotations.SortedTags(res))
+
+	return annotations.FindTagsResult{Tags: res}, nil
+}