[Usage Collection] Reuse concurrent collectors

[afharo]

Summary

This is part of our attempts to reduce the impact of Usage Collection in Kibana: This PR reuses the promise of an ongoing collector in case of a subsequent call comes in.

This is an extra layer of protection to the current refreshCache: false behavior. In some cases, we've seen explicit refreshCache: true calls that may overload Kibana if an expensive Usage Collector takes longer to process, and a new request comes in.

For maintainers

• This was checked for breaking API changes and was labeled appropriately

[kibana-ci]

💚 Build Succeeded

• Buildkite Build
• Commit: 5497ec3

Metrics [docs]

✅ unchanged

To update your PR or re-run it, just comment with:
@elasticmachine merge upstream

[elasticmachine]

Pinging @elastic/kibana-core (Team:Core)