Use NetworkTrafficAnnotation in URLRequestContext UMA logging

net/url_request/url_request_context.cc

Index: net/url_request/url_request_context.cc
diff --git a/net/url_request/url_request_context.cc b/net/url_request/url_request_context.cc
index da1330ea5bcc39c708ef21d64db05b41471fa870..52121e039ef4a6b157ebe72c138644e2e3c93638 100644
--- a/net/url_request/url_request_context.cc
+++ b/net/url_request/url_request_context.cc
@@ -5,6 +5,7 @@
 #include "net/url_request/url_request_context.h"
 
 #include <inttypes.h>
+#include <limits>
 #include <utility>
 
 #include "base/compiler_specific.h"
@@ -146,6 +147,10 @@ void URLRequestContext::InsertURLRequest(const URLRequest* request) const {
     largest_outstanding_requests_count_seen_ = url_requests_.size();
     UMA_HISTOGRAM_COUNTS_1M("Net.URLRequestContext.OutstandingRequests",
                             largest_outstanding_requests_count_seen_);
+    UMA_HISTOGRAM_ENUMERATION(
+        "Net.URLRequestContext.OutstandingRequests.Type",
+        request->traffic_annotation().unique_id_hash_code,
+        std::numeric_limits<base::HistogramBase::Sample>::max() - 1);

[mmenke, 2017/05/24 16:07:52]

Erm...wait...is this right?  SHould this be a sparse histogram or something?  I
think UMA_HISTOGRAM_ENUMERATION creates one bucket for every value?

[xunjieli, 2017/05/24 16:18:16]

I am not sure. I read the documentation on sparse histogram, but it says sparse
histogram is for histograms that are logged infrequently. Since
URLRequestContext::InsertURLRequest() is called for every network request, I am
not sure if that fits this use case.

+ asvitkine@
Alexei, I am adding a new histogram enum type, which is an unsigned int32. There
are about 100 of these values. What's the best way to log this data? 

[Alexei Svitkine (slow), 2017/05/24 17:13:29]

Do the values go outside of the signed int32 range? If not, the best way is to
use a sparse histogram. (UMA_HISTOGRAM_SPARSE()).

[xunjieli, 2017/05/24 17:23:02]

Thanks. 
+ rhalavati@, can we switch to signed int32 for hash code in your CL? 

   }
 }