Multi-strategy based tracking.

tools/metrics/histograms/histograms.xml

Index: tools/metrics/histograms/histograms.xml
diff --git a/tools/metrics/histograms/histograms.xml b/tools/metrics/histograms/histograms.xml
index e2baccf21567fdfb25d182588528aadf775ea7bf..b679237dbfd76fc08272333da1e6c6126cddb4dc 100644
--- a/tools/metrics/histograms/histograms.xml
+++ b/tools/metrics/histograms/histograms.xml
@@ -16766,6 +16766,13 @@ other types of suffix sets.
   </summary>
 </histogram>
 
+<histogram name="Settings.TrackedSplitPreferenceChangedCount">
+  <summary>
+    The number of items that had changed in a dictionary pref when
+    Settings.TrackedPreferenceChanged is reported for that pref.
+  </summary>
+</histogram>
+
 <histogram name="SettingsResetBubble.NumNoThanksPerReset">
   <summary>
     Counts the number of times the user clicked on the No Thanks button of the
@@ -32966,6 +32973,11 @@ other types of suffix sets.
   <affected-histogram name="Startup.BrowserOpenTabs"/>
 </fieldtrial>
 
+<fieldtrial name="TrackedSplitPreferences">
+  <group name="extension.settings" label="Extension IDs dictionary"/>
+  <affected-histogram name="Settings.TrackedSplitPreferenceChangedCount"/>

[Alexei Svitkine (slow), 2013/12/17 22:19:56]

This will result in the
"Settings.TrackedSplitPreferenceChangedCount_extensions.settings" histogram.

Hmm, I'm not sure that this is quite optimal, since the dot notation from the
pref name kind of clashes with the histogram notation and also exposes the
internal pref path, which isn't so great.

I think it should be possible to use the actual constant names (i.e.
kShowHomeButton) with some macro magic and making a field for it in the
TrackedPreference.

Basically, you can make a C macro that stringifys a name, so instead of
specifying:

 { 0, prefs::kShowHomeButton, TRACKING_STRATEGY_ATOMIC },

You can make a macro:

 TRACKED_PREF(0, kShowHomeButton, TRACKING_STRATEGY_ATOMIC),

Where the macro will then expand things to:

 { 0, "kShowHomeButton", prefs::kShowHomeButton, TRACKING_STRATEGY_ATOMIC },

And then you can use the string "kShowHomeButton" from the TrackedPreference
struct when logging this.

[gab, 2013/12/18 17:43:14]

Switched to using a '_' before the suffix instead of '.' in pref_hash_filter.cc
but I don't think avoiding the pref's name in the histogram is worth the
complexity of a macro here.

AFAIK dots after the first dot don't represent anything particular in histogram
names, right?

[Alexei Svitkine (slow), 2013/12/18 18:28:01]

I still prefer the macro approach (which actually isn't very complex to do), but
I'll let you make the call. If we're not going with that approach, I prefer that
this uses dots. For that, you can specifiy separator="." on this <fieldtrial>
(and change the other code back to using ".").

[gab, 2013/12/18 18:48:03]

Okay done (back to '.' before suffix + separator).

I don't agree that the macro is simple, e.g. it has to:
1) Care about the "prefs::" namespace (or otherwise keep it in the stringified
pref name)
2) Be public or tests can't build a proper array of TrackedPreferences.

and I don't think it matters at all that the histogram be suffixed after the
real pref name rather than its constant's name (either way the string is
hardcoded in histograms.xml so using the constant has no benefits IMO).

I try to avoid macros at all cost and I don't think the cost here is justified.

+</fieldtrial>
+
 <fieldtrial name="WebStoreLinkExperiment">
   <group name="Disabled" label="Neither extra webstore link is visible"/>
   <group name="FooterLink" label="Link in bottom right of footer"/>