The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/cc6943dbb0984ae198fa36706126d42a87a65d7b

commit cc6943dbb0984ae198fa36706126d42a87a65d7b
Author: jkarlin <jkarlin@chromium.org>
Date: Mon Apr 25 19:25:35 2016

Count successes of link rel=prefetch

Add metrics to count the number of bytes for requests fulfilled by the
cache that were filled by earlier prefetches. Also measures how long
those particular requests take (to compare against time taken for
prefetch network requests).

BUG= 606366 

Review URL: https://codereview.chromium.org/1921613003

Cr-Commit-Position: refs/heads/master@{#389534}

[modify] https://crrev.com/cc6943dbb0984ae198fa36706126d42a87a65d7b/content/browser/loader/resource_loader.cc
[modify] https://crrev.com/cc6943dbb0984ae198fa36706126d42a87a65d7b/net/url_request/url_request_http_job.cc
[modify] https://crrev.com/cc6943dbb0984ae198fa36706126d42a87a65d7b/tools/metrics/histograms/histograms.xml

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/cc6943dbb0984ae198fa36706126d42a87a65d7b

commit cc6943dbb0984ae198fa36706126d42a87a65d7b
Author: jkarlin <jkarlin@chromium.org>
Date: Mon Apr 25 19:25:35 2016

Count successes of link rel=prefetch

Add metrics to count the number of bytes for requests fulfilled by the
cache that were filled by earlier prefetches. Also measures how long
those particular requests take (to compare against time taken for
prefetch network requests).

BUG= 606366 

Review URL: https://codereview.chromium.org/1921613003

Cr-Commit-Position: refs/heads/master@{#389534}

[modify] https://crrev.com/cc6943dbb0984ae198fa36706126d42a87a65d7b/content/browser/loader/resource_loader.cc
[modify] https://crrev.com/cc6943dbb0984ae198fa36706126d42a87a65d7b/net/url_request/url_request_http_job.cc
[modify] https://crrev.com/cc6943dbb0984ae198fa36706126d42a87a65d7b/tools/metrics/histograms/histograms.xml

Ah, this is unfortunately not as easy as I thought. unused_since_prefetch is true if the cache was filled by a network request or if it was already in the cache. So the UMA I just added is still useful (to measure how often we use prefetched resources) but doesn't indicate how many bytes prefetched over the network were used.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/409db34744b555667993ce3c7eed40e7369582da

commit 409db34744b555667993ce3c7eed40e7369582da
Author: jkarlin <jkarlin@chromium.org>
Date: Fri Apr 29 20:34:04 2016

Clarify the summary of some prefetch histograms.

BUG= 606366 

Review-Url: https://codereview.chromium.org/1938523002
Cr-Commit-Position: refs/heads/master@{#390742}

[modify] https://crrev.com/409db34744b555667993ce3c7eed40e7369582da/tools/metrics/histograms/histograms.xml

You started fixing this bug over two years ago. Are you still working on it? 

I haven't gotten around to this and haven't had a need for it so wontfixing it.