[8.x](backport #4057) Redact APM API key and secret token

[mergify]

What is the problem this PR solves?

APM secrets were visible in elastic-agent diagnostics

How does this PR solve the problem?

Included APM API key and secret token to the redactServer() function so that any non-empty values are subtituted by [redacted]

Design Checklist

• I have ensured my design is stateless and will work when multiple fleet-server instances are behind a load balancer.
• I have or intend to scale test my changes, ensuring it will work reliably with 100K+ agents connected.
• I have included fail safe mechanisms to limit the load on fleet-server: rate limiting, circuit breakers, caching, load shedding, etc.

Checklist

• I have commented my code, particularly in hard-to-understand areas
• I have added tests that prove my fix is effective or that my feature works

Related issues

How to test this PR

• Spin up a 8.16.0 ESS deployment
• Create a diagnostics archive for the agent running Fleet and APM server (the one with the Elastic Cloud policy assigned)
• Check that in /components/fleet-server-es-containerhost/fleet-server.yml under inputs[0]/instrumentation the secret token is not shown in clear

  ---

  This is an automatic backport of pull request Redact APM API key and secret token #4057 done by Mergify.

[elastic-sonarqube]

Quality Gate passed

Issues
0 New issues
0 Fixed issues
0 Accepted issues

Measures
0 Security Hotspots
100.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube