Add histograms to learn about metrics files dropped for lack of an embedded profile.

components/metrics/file_metrics_provider.cc

Index: components/metrics/file_metrics_provider.cc
diff --git a/components/metrics/file_metrics_provider.cc b/components/metrics/file_metrics_provider.cc
index 61297c6e32b198084f64e80e1f2d339cbb8c3b6f..cfc38ff73ee8161264059d7ef00e945cc4eb63ba 100644
--- a/components/metrics/file_metrics_provider.cc
+++ b/components/metrics/file_metrics_provider.cc
@@ -546,6 +546,30 @@ bool FileMetricsProvider::ProvideIndependentMetrics(
       RecordEmbeddedProfileResult(EMBEDDED_PROFILE_FOUND);
     } else {
       RecordEmbeddedProfileResult(EMBEDDED_PROFILE_DROPPED);
+
+      // TODO(bcwhite): Remove these once crbug/695880 is resolved.
+
+      int histogram_count = 0;
+      base::PersistentHistogramAllocator::Iterator histogram_iter(
+          source->allocator.get());
+      while (true) {
+        std::unique_ptr<base::HistogramBase> histogram =
+            histogram_iter.GetNext();
+        if (!histogram)
+          break;
+        ++histogram_count;
+      }

[Alexei Svitkine (slow), 2017/06/30 14:50:10]

Nit: How about:

while (histogram_iter.GetNext()) {
  ++histogram_count;
}

[bcwhite, 2017/06/30 15:12:15]

Done.

+      UMA_HISTOGRAM_COUNTS_1000(

[Alexei Svitkine (slow), 2017/06/30 14:50:10]

Nit: Suggest using 10k version of the macro because I *think* there could be
more than 1000.

[bcwhite, 2017/06/30 15:12:16]

Done.  It doesn't really matter either way.  If it's more than 1000, you know
you have a full run and not an early crash that just didn't get around to
writing a system profile.

+          "UMA.FileMetricsProvider.EmbeddedProfile.DroppedHistogramCount",
+          histogram_count);
+
+      base::File::Info info;
+      if (base::GetFileInfo(source->path, &info)) {
+        UMA_HISTOGRAM_CUSTOM_TIMES(
+            "UMA.FileMetricsProvider.EmbeddedProfile.DroppedFileAge",
+            base::Time::Now() - info.last_modified,
+            base::TimeDelta::FromSeconds(1), base::TimeDelta::FromDays(1), 100);

[Alexei Svitkine (slow), 2017/06/30 14:50:10]

I think logging in ms is not useful here, so you don't need to use CUSTOM_TIMES.

Also 1 day seems like too low a cap.

Suggest minute granularity, 50 buckets and let's say 30 days as max.

i.e. similar to what we do for seed freshness:

    UMA_HISTOGRAM_CUSTOM_COUNTS("Variations.SeedFreshness", delta.InMinutes(),
        1, base::TimeDelta::FromDays(30).InMinutes(), 50);

[bcwhite, 2017/06/30 15:12:16]

Done. 

+      }
     }
 
     // Regardless of whether this source was successfully recorded, it is never