Documented parameters to DropTargetLabels (#120)

The rationale was buried in a discussion thread in the original PR, and the reasoning isn't obvious until you understand exactly how Target.Labels and MonitoredResource are constructed: #15 (comment)

Author: jkohen

retrieval/series_cache.go

@@ -337,6 +337,9 @@ func (c *seriesCache) refresh(ctx context.Context, ref uint64) error {
 		return nil
 	}
 	// Remove target labels and __name__ label.
+	// Stackdriver only accepts a limited amount of labels, so we choose to economize aggressively here. This should be OK
+	// because we expect that the target.Labels will be redundant with the Stackdriver MonitoredResource, which is derived
+	// from the target Labels and DiscoveredLabels.
 	finalLabels := targets.DropTargetLabels(pkgLabels(entry.lset), target.Labels)
 	for i, l := range finalLabels {
 		if l.Name == "__name__" {