Follow-up of bug 1786249.
There might be cases where sponsored visits can not find a triggering URL.
In this bug, we add telemetry to get the number of such visits.

LGTM, please attach an appropriate data-review request and get it reviewed

Comment on attachment 9292363 [details]
data-review-request-for-bug1787961.md

1. Is there or will there be documentation that describes the schema for the ultimate data set in a public, complete, and accurate way?
   Yes, this will be documented in the probe dictionary.

2. Is there a control mechanism that allows the user to turn the data collection on and off?
   Yes, users can opt out of data collection.

3. If the request is for permanent data collection, is there someone who will monitor the data over time?
   The search team will monitor over time.

4. Using the category system of data types on the Mozilla wiki, what collection type of data do the requested measurements fall under?
   Category 2, Interaction data.

5. Is the data collection request for default-on or default-off?
   Default on.

6. Does the instrumentation include the addition of any new identifiers?
   No new identifiers.

7. Is the data collection covered by the existing Firefox privacy notice?
   Yes.

8. Does the data collection use a third-party collection tool?
   No.

data-review +

https://hg.mozilla.org/mozilla-central/rev/18c35bc1fd45