From meeting with PM, we'd like metrics for:
- Usage time (over all clients).
- Usage counts (to compare with CCTs).
- Verification Outcome (including if the verification was cached, this will give us an insight into how often TWAs are used offline).
- Maybe UKM for clients (so we can see how many developers are using TWAs/what the most popular TWAs are).

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/39de8ed178a3a57bd7bb9032dc562dff9d7fd35b

commit 39de8ed178a3a57bd7bb9032dc562dff9d7fd35b
Author: Peter E Conn <peconn@chromium.org>
Date: Thu May 17 10:39:50 2018

🔁 Add UMA for Trusted Web Activity verification.

Bug:  813104 
Change-Id: I53b9abc17c9af393d6ffaa84203ab0b24ab7ecb9
Reviewed-on: https://chromium-review.googlesource.com/1061517
Reviewed-by: Robert Kaplow <rkaplow@chromium.org>
Reviewed-by: Bernhard Bauer <bauerb@chromium.org>
Commit-Queue: Peter Conn <peconn@chromium.org>
Cr-Commit-Position: refs/heads/master@{#559486}
[add] https://crrev.com/39de8ed178a3a57bd7bb9032dc562dff9d7fd35b/chrome/android/java/src/org/chromium/chrome/browser/browserservices/BrowserServicesMetrics.java
[modify] https://crrev.com/39de8ed178a3a57bd7bb9032dc562dff9d7fd35b/chrome/android/java/src/org/chromium/chrome/browser/browserservices/OriginVerifier.java
[modify] https://crrev.com/39de8ed178a3a57bd7bb9032dc562dff9d7fd35b/chrome/android/java_sources.gni
[modify] https://crrev.com/39de8ed178a3a57bd7bb9032dc562dff9d7fd35b/tools/metrics/histograms/enums.xml
[modify] https://crrev.com/39de8ed178a3a57bd7bb9032dc562dff9d7fd35b/tools/metrics/histograms/histograms.xml

Bernhard, with regards to the UKM are we talking about Rappor here? I'm not sure it's suitable (specifically the expectation of receiving >500,000 reports per time interval).

No, see go/ukm and go/ukm-api.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/5bc81b5837cd658c6d8b9fdccb56d0406527b3d9

commit 5bc81b5837cd658c6d8b9fdccb56d0406527b3d9
Author: Peter E Conn <peconn@chromium.org>
Date: Mon Jun 04 09:43:14 2018

🔁 Collect UKM when verification succeeds for a TWA.

Collect an User Keyed Metric for when verification succeeds for a
Trusted Web Activity. This will give us info on what sites TWAs are
used for.

Bug:  813104 
Change-Id: Id391610553209baa027e5e32d80f7b25a4944f65
Reviewed-on: https://chromium-review.googlesource.com/1082444
Commit-Queue: Peter Conn <peconn@chromium.org>
Reviewed-by: Robert Kaplow <rkaplow@chromium.org>
Reviewed-by: Bernhard Bauer <bauerb@chromium.org>
Reviewed-by: Benoit L <lizeb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#564036}
[add] https://crrev.com/5bc81b5837cd658c6d8b9fdccb56d0406527b3d9/chrome/android/java/src/org/chromium/chrome/browser/browserservices/UkmRecorder.java
[modify] https://crrev.com/5bc81b5837cd658c6d8b9fdccb56d0406527b3d9/chrome/android/java/src/org/chromium/chrome/browser/webapps/WebappActivity.java
[modify] https://crrev.com/5bc81b5837cd658c6d8b9fdccb56d0406527b3d9/chrome/android/java_sources.gni
[modify] https://crrev.com/5bc81b5837cd658c6d8b9fdccb56d0406527b3d9/chrome/browser/BUILD.gn
[add] https://crrev.com/5bc81b5837cd658c6d8b9fdccb56d0406527b3d9/chrome/browser/android/browserservices/ukm_recorder.cc
[modify] https://crrev.com/5bc81b5837cd658c6d8b9fdccb56d0406527b3d9/tools/metrics/ukm/ukm.xml

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/751d79d77b6f6cacf2cd80006a1dbcaef59165a8

commit 751d79d77b6f6cacf2cd80006a1dbcaef59165a8
Author: Peter E Conn <peconn@chromium.org>
Date: Thu Jun 07 18:57:23 2018

🔁 Add last bits of UMA for TWAs.

Bug:  813104 
Change-Id: I0139374f0f05c2a685d413e8c9d4bfcd5204cd67
Reviewed-on: https://chromium-review.googlesource.com/1088702
Commit-Queue: Peter Conn <peconn@chromium.org>
Reviewed-by: Bernhard Bauer <bauerb@chromium.org>
Reviewed-by: Robert Kaplow <rkaplow@chromium.org>
Cr-Commit-Position: refs/heads/master@{#565366}
[modify] https://crrev.com/751d79d77b6f6cacf2cd80006a1dbcaef59165a8/chrome/android/java/src/org/chromium/chrome/browser/browserservices/BrowserServicesMetrics.java
[modify] https://crrev.com/751d79d77b6f6cacf2cd80006a1dbcaef59165a8/chrome/android/java/src/org/chromium/chrome/browser/webapps/WebappActivity.java
[modify] https://crrev.com/751d79d77b6f6cacf2cd80006a1dbcaef59165a8/tools/metrics/actions/actions.xml
[modify] https://crrev.com/751d79d77b6f6cacf2cd80006a1dbcaef59165a8/tools/metrics/histograms/histograms.xml