[Lens] Add telemetry for field list available/emptiness

[flash1293]

As described in #95558, index filter on the field caps API should be the primary way of filtering down field lists to the relevant sub set.

However in Lens document sampling is already used for this purpose. To make sure the experience doesn't get worse when switching over, one index filters on the field caps API are exposed and integrated in Kibana, telemetry should be added to the Lens field list to determine how many fields are exclude based on document sampling vs field caps.

Open questions:

• Which metric exactly should be recorded?

[elasticmachine]

Pinging @elastic/kibana-app (Team:KibanaApp)

[flash1293]

With the decision made in #112782 this is now unnecessary