…s metrics (elastic#173001)

Closes: elastic#170031
Replaces: elastic/elasticsearch#101467

**Problem**
We need to know if an agent config has been applied at the edge (by APM
agents). This is determined by comparing the etag (hash) of the config,
with the etag applied at the edges.

Previously the agent config itself contained this information
(`config.applied_by_agent`) but when running with fleet this will
instead be captured in `agent_config` metric documents.

Currently the internal kibana user retrieves the `agent_config` metric
documents from the APM metric index (`metrics-apm-*` by default). This
index is configurable by the end-user so can be changed to something the
internal user doesn't have access to. This is a problem.

**Solution**

This PR replaces the calls made by the internal client with calls made
by the authenticated end user (via `APMEventClient`). This approach
works for requests made from the browser/UI but doesn't work for
background tasks made by fleet. To work around this we only
conditionally query the metric index if the `APMEventClient` is
available.
If `APMEventClient` is not available `applied_by_agent` will be
`undefined`

(cherry picked from commit 58c7958)