fix: register kept_from_stress metric (#1487)

## Which problem is this PR solving?

register `kept_from_stress` metric

## Short description of the changes

- register `kept_from_stress` metric

collect/collect.go

@@ -150,6 +150,7 @@ var inMemCollectorMetrics = []metrics.Metadata{
 	{Name: TraceSendLateSpan, Type: metrics.Counter, Unit: metrics.Dimensionless, Description: "number of spans that are sent due to late span arrival"},
 
 	{Name: "dropped_from_stress", Type: metrics.Counter, Unit: metrics.Dimensionless, Description: "number of spans dropped due to stress relief"},
+	{Name: "kept_from_stress", Type: metrics.Counter, Unit: metrics.Dimensionless, Description: "number of spans kept due to stress relief"},
 	{Name: "trace_kept_sample_rate", Type: metrics.Histogram, Unit: metrics.Dimensionless, Description: "sample rate of kept traces"},
 	{Name: "trace_aggregate_sample_rate", Type: metrics.Histogram, Unit: metrics.Dimensionless, Description: "aggregate sample rate of both kept and dropped traces"},
 	{Name: "collector_redistribute_traces_duration_ms", Type: metrics.Histogram, Unit: metrics.Milliseconds, Description: "duration of redistributing traces to peers"},