Add sparse histograms to the metrics library and metrics client.

Samples to these histograms are any 32-bit int value.

BUG=chromium:222189
TEST=manual

Change-Id: Ic8d5773d05d717a275c4a4b5616e0e4c307337b8
Reviewed-on: https://gerrit.chromium.org/gerrit/45897
Tested-by: Luigi Semenzato <semenzato@chromium.org>
Reviewed-by: Darin Petkov <petkov@chromium.org>
Commit-Queue: Luigi Semenzato <semenzato@chromium.org>
Reviewed-by: Luigi Semenzato <semenzato@chromium.org>
diff --git a/metrics/c_metrics_library.h b/metrics/c_metrics_library.h
index 5c7003d..28ae916 100644
--- a/metrics/c_metrics_library.h
+++ b/metrics/c_metrics_library.h
@@ -28,6 +28,10 @@
 int CMetricsLibrarySendEnumToUMA(CMetricsLibrary handle,
                                  const char* name, int sample, int max);
 
+// C wrapper for MetricsLibrary::SendSparseToUMA.
+int CMetricsLibrarySendSparseToUMA(CMetricsLibrary handle,
+                                   const char* name, int sample);
+
 // C wrapper for MetricsLibrary::SendUserActionToUMA.
 int CMetricsLibrarySendUserActionToUMA(CMetricsLibrary handle,
                                        const char* action);