Fix TrendSink median calculation as part of Thresholds.Run

This commit addresses the issues outlined in grafana#2755.

We assume that because the trend sink median computation can turn out
to be quite expensive (it involves sorting the array of values), it was
so far delegated to the last moment: thresholds evaluation and summary
generation. This detail appeared to be missed in the most recent
thresholds refactoring, and the `Calc` method was never used by the
`Thresholds.Run` method.

This commit ensures that in the same fashion as with Trend Sinks
percentiles, the thresholds evaluation also computes the sink's median
value when necessary.

This commit adds dedicated tests illustrating the behavior, and
demonstrating that it solves said issue.

metrics/thresholds.go

@@ -188,7 +188,7 @@ func (ts *Thresholds) Run(sink Sink, duration time.Duration) (bool, error) {
 		ts.sinked["min"] = sinkImpl.Min
 		ts.sinked["max"] = sinkImpl.Max
 		ts.sinked["avg"] = sinkImpl.Avg
-		ts.sinked["med"] = sinkImpl.Med
+		ts.sinked["med"] = sinkImpl.P(0.5)
 
 		// Parse the percentile thresholds and insert them in
 		// the sinks mapping.

metrics/thresholds_test.go

@@ -638,12 +638,13 @@ func TestThresholdsRunAll(t *testing.T) {
 	}
 }
 
-func TestThresholds_Run(t *testing.T) {
+func TestThresholdsRun(t *testing.T) {
 	t.Parallel()
 
 	type args struct {
-		sink     Sink
-		duration time.Duration
+		sink                 Sink
+		thresholdExpressions []string
+		duration             time.Duration
 	}
 	tests := []struct {
 		name    string
@@ -652,30 +653,83 @@ func TestThresholds_Run(t *testing.T) {
 		wantErr bool
 	}{
 		{
-			name:    "Running thresholds of existing sink",
-			args:    args{DummySink{"p(95)": 1234.5}, 0},
+			name: "Running thresholds of existing sink",
+			args: args{
+				sink:                 DummySink{"p(95)": 1234.5},
+				thresholdExpressions: []string{"p(95)<2000"},
+				duration:             0,
+			},
+			want:    true,
+			wantErr: false,
+		},
+		{
+			name: "Running thresholds of existing sink but failing threshold",
+			args: args{
+				sink:                 DummySink{"p(95)": 3000},
+				thresholdExpressions: []string{"p(95)<2000"},
+				duration:             0,
+			},
+			want:    false,
+			wantErr: false,
+		},
+		{
+			name: "Running threshold on non existing sink does not fail",
+			args: args{
+				sink:                 DummySink{"dummy": 0},
+				thresholdExpressions: []string{"p(95)<2000"},
+				duration:             0,
+			},
+			want:    true,
+			wantErr: false,
+		},
+		{
+			name: "Running threshold on trend sink with no values and passing med statement succeeds",
+			args: args{
+				sink:                 &TrendSink{Values: []float64{}, Med: 0},
+				thresholdExpressions: []string{"med<39"},
+				duration:             0,
+			},
 			want:    true,
 			wantErr: false,
 		},
 		{
-			name:    "Running thresholds of existing sink but failing threshold",
-			args:    args{DummySink{"p(95)": 3000}, 0},
+			name: "Running threshold on trend sink with no values and non passing med statement fails",
+			args: args{
+				sink:                 &TrendSink{Values: []float64{}, Med: 0},
+				thresholdExpressions: []string{"med>39"},
+				duration:             0,
+			},
 			want:    false,
 			wantErr: false,
 		},
 		{
-			name:    "Running threshold on non existing sink does not fail",
-			args:    args{DummySink{"dummy": 0}, 0},
+			name: "Running threshold on trend sink with values and passing med statement succeeds",
+			args: args{
+				sink:                 &TrendSink{Values: []float64{70, 80, 90}, Count: 3},
+				thresholdExpressions: []string{"med>39"},
+				duration:             0,
+			},
 			want:    true,
 			wantErr: false,
 		},
+		{
+			name: "Running threshold on trend sink with values and failing med statement fails",
+			args: args{
+				sink:                 &TrendSink{Values: []float64{70, 80, 90}, Count: 3},
+				thresholdExpressions: []string{"med<39"},
+				duration:             0,
+			},
+			want:    false,
+			wantErr: false,
+		},
 	}
 	for _, testCase := range tests {
 		testCase := testCase
+
 		t.Run(testCase.name, func(t *testing.T) {
 			t.Parallel()
 
-			thresholds := NewThresholds([]string{"p(95)<2000"})
+			thresholds := NewThresholds(testCase.args.thresholdExpressions)
 			gotParseErr := thresholds.Parse()
 			require.NoError(t, gotParseErr)