Extract interface from ActivityQueryTracker and allows passing custom implementation (#102)

* Extract QueryTracker interface.

* Update from Prometheus PR 10071.

* Update promql/engine.go

Co-authored-by: Mauro Stettler <mauro.stettler@gmail.com>

* Update promql/engine.go

Co-authored-by: Mauro Stettler <mauro.stettler@gmail.com>

Co-authored-by: Mauro Stettler <mauro.stettler@gmail.com>

promql/engine.go

@@ -207,13 +207,29 @@ func contextErr(err error, env string) error {
 	}
 }
 
+// QueryTracker provides access to two features:
+// 1) tracking of active queries that are logged on restart if PromQL engine crashes while executing the query, and
+// 2) enforcement of the maximum number of concurrent requests.
+type QueryTracker interface {
+	// GetMaxConcurrent returns maximum number of concurrent queries that are allowed by this tracker.
+	GetMaxConcurrent() int
+
+	// Insert inserts query into query tracker. This call must block if maximum number of queries is already running.
+	// If Insert doesn't return error then returned integer value should be used in subsequent Delete call.
+	// Insert should return error if context is finished before query can proceed, and integer value returned in this case should be ignored by caller.
+	Insert(ctx context.Context, query string) (int, error)
+
+	// Delete removes query from activity tracker. InsertIndex is value returned by Insert call.
+	Delete(insertIndex int)
+}
+
 // EngineOpts contains configuration options used when creating a new Engine.
 type EngineOpts struct {
 	Logger             log.Logger
 	Reg                prometheus.Registerer
 	MaxSamples         int
 	Timeout            time.Duration
-	ActiveQueryTracker *ActiveQueryTracker
+	ActiveQueryTracker QueryTracker
 	// LookbackDelta determines the time since the last sample after which a time
 	// series is considered stale.
 	LookbackDelta time.Duration
@@ -236,7 +252,7 @@ type Engine struct {
 	metrics                  *engineMetrics
 	timeout                  time.Duration
 	maxSamplesPerQuery       int
-	activeQueryTracker       *ActiveQueryTracker
+	activeQueryTracker       QueryTracker
 	queryLogger              QueryLogger
 	queryLoggerLock          sync.RWMutex
 	lookbackDelta            time.Duration