tests: Add parallel monitoring middleware test

[1pkg]

Motivation/summary

This PR adds concurrency test for the fix from #14335.

Checklist

• Update CHANGELOG.asciidoc
• Documentation has been updated

For functional changes, consider:

• Is it observable through the addition of either logging or metrics?
• Is its use being published in telemetry to enable product improvement?
• Have system tests been added to avoid regression?

How to test these changes

Test results from v8.15.2

=== RUN   TestMonitoringHandler
=== RUN   TestMonitoringHandler/Parallel
==================
WARNING: DATA RACE
Read at 0x00c000127050 by goroutine 24:
  runtime.mapassign_fast64ptr()
 ...
 ...
 ...

Previous write at 0x00c000127050 by goroutine 23:
  runtime.mapaccess2_faststr()
 ...
 ...
 ...

Goroutine 24 (running) created at:
 ...
 ...
 ...

Goroutine 23 (finished) created at:
 ...
 ...
 ...
==================

Test results from main

=== RUN   TestMonitoringHandler
=== RUN   TestMonitoringHandler/Parallel
--- PASS: TestMonitoringHandler (0.00s)
    --- PASS: TestMonitoringHandler/Parallel (0.00s)
PASS
ok  	github.com/elastic/apm-server/internal/beater/middleware	1.547s

Related issues

#14335