[beats receivers] Enable beats receivers for internal monitoring data collection by default

[cmacknz]

Enable beats receivers for internal monitoring data collection by default on main and in integration tests so we can start soak testing it in development and test environments.

Draft PR showing some unit test failures to address:

• Enable beats receivers for monitoring by default. #9037

That PR also need to change the existing monitoring data comparison test to start with beats receivers off (or just reverse the test order where they start on and are then turned off): https://github.com/cmacknz/elastic-agent/blob/184b0584efc99fd8f3ad9dc1a33018a44a8f726a/testing/integration/ess/beat_receivers_test.go#L43

[elasticmachine]

Pinging @elastic/elastic-agent-control-plane (Team:Elastic-Agent-Control-Plane)

[swiatekm]

I've started checking how our integration tests react to the change in #10204. Thus far, I've had to:

• Modify the golden file for generated self-monitoring config. We should probably have two of these, separately for running in process and otel mode.
• The runtime leak test needs to detect running agent subprocesses differently. It should probably just look at child processes instead of using status, because right now it wouldn't detect us leaking an otel process. See 0f3b193.
• Our fixtures check if agent was successfully uninstalled by looking at running process commands. As we start the otel collector by calling elastic-agent otel, it gets detected as another agent. Fix here only counts agents who aren't children of other agents, although this should eventually become irrelevant when we split the binaries.
• TBD, some tests are still failing.

Overall, I don't see any semantic failures yet, just assertions with implicit assumptions that aren't true anymore.