Nonexistent labels in BY-clauses shouldn't propagate to result.

This fixes bug 2. of https://github.com/prometheus/prometheus/issues/374

Change-Id: Ia4a13153616bafce5bf10597966b071434422d09

rules/ast/ast.go

@@ -398,7 +398,9 @@ func (node *VectorAggregation) Eval(timestamp clientmodel.Timestamp, view *viewA
 			} else {
 				m[clientmodel.MetricNameLabel] = sample.Metric[clientmodel.MetricNameLabel]
 				for _, l := range node.groupBy {
-					m[l] = sample.Metric[l]
+					if v, ok := sample.Metric[l]; ok {
+						m[l] = v
+					}
 				}
 			}
 			result[groupingKey] = &groupedAggregation{

rules/rules_test.go

@@ -139,6 +139,15 @@ func TestExpressions(t *testing.T) {
 			},
 			fullRanges:     0,
 			intervalRanges: 8,
+		}, {
+			// Non-existent labels mentioned in BY-clauses shouldn't propagate to output.
+			expr: `SUM(http_requests) BY (job, nonexistent)`,
+			output: []string{
+				`http_requests{job="api-server"} => 1000 @[%v]`,
+				`http_requests{job="app-server"} => 2600 @[%v]`,
+			},
+			fullRanges:     0,
+			intervalRanges: 8,
 		}, {
 			expr: `
 				// Test comment.