[Synthetics] Provide feedback on the number of monitors covered by rule definitions

[justinkambic]

Describe the feature:

As a user, when I'm creating a Monitor Status or TLS rule, I want to know how many monitors will be covered by the conditions I specify in the flyout.

Describe a specific use case for the feature:

This is a feature parity issue with legacy Uptime UI vs. Synthetics; we should give the user the feedback that X monitors are included in the query they're building via our filtering UI. A further addition to this feature would be to create the capability to see the names of the monitors impacted, but this is not something we have offered in the past.

[elasticmachine]

Pinging @elastic/obs-ux-management-team (Team:obs-ux-management)