Links to looker explores should not reference client count for pings without a client id

[wlach]

A good example would be the explore for the background update ping: https://mozilla.cloud.looker.com/explore/firefox_desktop_background_update/background_update?qid=XMTEmtkFtlBVT9sCPQ9pSh&toggle=fil,vis

The client count has no meaning, because a client id is not sent with these pings:

https://dictionary.telemetry.mozilla.org/apps/firefox_desktop_background_update/pings/background-update

For these cases, we should specify the ping count measure instead of client count. The offending line is here:

glean-dictionary/scripts/build-glean-metadata.py

Line 429 in c7a542e

f"{ping_name_snakecase}.clients",

However, you will need to find out if the ping that a metric is a part of includes a client identifier for each case (note that metrics might be sent by more than one ping, some of which may have a client identifier and some of which may not).

@Iinh can you take this one when you have time? This is more of an edge case than #871 but we should still fix it.

See also: mozilla/lookml-generator#253