[Telemetry] Fix histogram naming.
[Telemetry] Fix histogram naming.

trace_tags should show up on the trace names but not the chart names. Previously
we'd output:
HISTOGRAM V8.MemoryHeapSampleTotalUsed_foo: V8.MemoryHeapSampleTotalUsed_foo=

Now we output:
HISTOGRAM V8.MemoryHeapSampleTotalUsed: V8.MemoryHeapSampleTotalUsed_foo=

This prevents an explosion of charts as seen on:
http://build.chromium.org/f/chromium/perf/linux-release/moz/report.html?history=150&rev=-1&graph=V8.MemoryExternalFragmentationTotal_extcs1_0.5

BUG=196411
TEST=tools/perf/run_multipage_benchmarks --browser=system page_cycler tools/perf/page_sets/page_cycler/moz.json --output-trace-tag=_foo
NOTRY=True


Review URL: https://chromiumcodereview.appspot.com/12900005

git-svn-id: de016e52bd170d2d4f2344f9bf92d50478b649e0@189022 0039d316-1c4b-4281-b951-d872f2087c98

benschmaus/catapult,sahiljain/catapult,catapult-project/catapult-csm,catapult-project/catapult-csm,catapult-project/catapult-csm,catapult-project/catapult,SummerLW/Perf-Insight-Report,catapult-project/catapult-csm,benschmaus/catapult,benschmaus/catapult,catapult-project/catapult,sahiljain/catapult,catapult-project/catapult-csm,sahiljain/catapult,catapult-project/catapult,sahiljain/catapult,benschmaus/catapult,sahiljain/catapult,benschmaus/catapult,catapult-project/catapult,catapult-project/catapult-csm,catapult-project/catapult,catapult-project/catapult,catapult-project/catapult-csm,benschmaus/catapult,SummerLW/Perf-Insight-Report,catapult-project/catapult,benschmaus/catapult,SummerLW/Perf-Insight-Report,SummerLW/Perf-Insight-Report,SummerLW/Perf-Insight-Report,sahiljain/catapult,SummerLW/Perf-Insight-Report