[APM] Service Inventory: Display count of services that are not in the selected environment

[formgeist]

Summary

Related to #77747

We should think about a way to describe to the user that the Service inventory isn't showing all available services because the environment selector has a specific environment variable selected. I.e. "user sets up a new service with environment dev, where is my service?`

[elasticmachine]

Pinging @elastic/apm-ui (Team:apm)

[formgeist]

@sqren This should be moving with #77747 so I've moved this and added it to the design planned items so we can have a solution for this soon-ish.

[botelastic]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.

[formgeist]

The use-case for adding this summary to the list has been updated to support the default environment option mentioned in #126100. Additionally, we discussed another use-case in our product design weekly that users would always want to have a top-level aggregate of all the services available in the cluster for a solution owner to monitor the instrumentation of APM in their applications. cc @akhileshpok @chrisdistasio for your consideration

[dgieselaar]

Displaying a count of services for other environments requires us to iterate over all of the data (possibly with the exclusion of the currently selected environment, but that depends on where we should be getting the "of n services" count from).

That doesn't make it a blocker, but it is something to keep in mind.