Loki: Set Query limits context header (#114112)

* feat(lokiQueryLimitsContext): send full query range & expr on initial split requests

pkg/tsdb/loki/kinds/dataquery/types_dataquery_gen.go

@@ -18,6 +18,17 @@ const (
 	QueryEditorModeBuilder QueryEditorMode = "builder"
 )
 
+type LimitsContext struct {
+	Expr string `json:"expr"`
+	From int64  `json:"from"`
+	To   int64  `json:"to"`
+}
+
+// NewLimitsContext creates a new LimitsContext object.
+func NewLimitsContext() *LimitsContext {
+	return &LimitsContext{}
+}
+
 type LokiQueryType string
 
 const (
@@ -59,6 +70,8 @@ type LokiDataQuery struct {
 	Instant *bool `json:"instant,omitempty"`
 	// Used to set step value for range queries.
 	Step *string `json:"step,omitempty"`
+	// The full query plan for split/shard queries. Encoded and sent to Loki via `X-Loki-Query-Limits-Context` header. Requires "lokiQueryLimitsContext" feature flag
+	LimitsContext *LimitsContext `json:"limitsContext,omitempty"`
 	// A unique identifier for the query within the list of targets.
 	// In server side expressions, the refId is used as a variable name to identify results.
 	// By default, the UI will assign A->Z; however setting meaningful names may be useful.