Looker explores for glean pings shouldn't have client count measure if ping doesn't have a client count

[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

We should hide this measure for this case.

[wlach]

We should hide this measure for this case.

Prior art for hiding a measure: #233

Although now that I think about it, a better solution would be to not generate the measure at all, rather than hiding it.