Elasticsearch: Support nested aggregation (#62301)

* Add nested query support

* Add nested support for alerts

* update nested aggregation

* cleanup types

* Add nested integration test

* Move aggdef to nested

* fixed merge conflict

* fixed lint warning

* mark nested-mode experimental

---------

Co-authored-by: Ethan Gallant <ethan@ziax.com>
Co-authored-by: Ethan J. Gallant <ethan.gallant@acquia.com>

pkg/tsdb/elasticsearch/client/models.go

@@ -236,6 +236,11 @@ type TermsAggregation struct {
 	Missing     *string                `json:"missing,omitempty"`
 }
 
+// NestedAggregation represents a nested aggregation
+type NestedAggregation struct {
+	Path string `json:"path"`
+}
+
 // ExtendedBounds represents extended bounds
 type ExtendedBounds struct {
 	Min int64 `json:"min"`

pkg/tsdb/elasticsearch/client/search_request.go

@@ -270,6 +270,7 @@ type AggBuilder interface {
 	Histogram(key, field string, fn func(a *HistogramAgg, b AggBuilder)) AggBuilder
 	DateHistogram(key, field string, fn func(a *DateHistogramAgg, b AggBuilder)) AggBuilder
 	Terms(key, field string, fn func(a *TermsAggregation, b AggBuilder)) AggBuilder
+	Nested(key, path string, fn func(a *NestedAggregation, b AggBuilder)) AggBuilder
 	Filters(key string, fn func(a *FiltersAggregation, b AggBuilder)) AggBuilder
 	GeoHashGrid(key, field string, fn func(a *GeoHashGridAggregation, b AggBuilder)) AggBuilder
 	Metric(key, metricType, field string, fn func(a *MetricAggregation)) AggBuilder
@@ -382,6 +383,26 @@ func (b *aggBuilderImpl) Terms(key, field string, fn func(a *TermsAggregation, b
 	return b
 }
 
+func (b *aggBuilderImpl) Nested(key, field string, fn func(a *NestedAggregation, b AggBuilder)) AggBuilder {
+	innerAgg := &NestedAggregation{
+		Path: field,
+	}
+	aggDef := newAggDef(key, &aggContainer{
+		Type:        "nested",
+		Aggregation: innerAgg,
+	})
+
+	if fn != nil {
+		builder := newAggBuilder()
+		aggDef.builders = append(aggDef.builders, builder)
+		fn(innerAgg, builder)
+	}
+
+	b.aggDefs = append(b.aggDefs, aggDef)
+
+	return b
+}
+
 func (b *aggBuilderImpl) Filters(key string, fn func(a *FiltersAggregation, b AggBuilder)) AggBuilder {
 	innerAgg := &FiltersAggregation{
 		Filters: make(map[string]interface{}),