apmpackage: disallow multiple policies

[axw]

Motivation/summary

Prevent the APM integration from being added multiple times to an agent policy.

Requires #4693

How to test these changes

1. Install apm integration
2. Add apm integration to an agent policy
3. Attempt to add another integration to the agent policy, apm integration will no longer be an option

Related issues

Closes #4539

[apmmachine]

💚 Build Succeeded

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Build stats

• Build Cause: Pull request #4694 updated

• Start Time: 2021-02-09T06:02:51.590+0000

• Duration: 42 min 27 sec

• Commit: 2e312ef

Test stats 🧪

Test	Results
Failed	0
Passed	4628
Skipped	124
Total	4752

Trends 🧪

Steps errors

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Compress

• Took 0 min 0 sec . View more details on here
• Description: tar --exclude=coverage-files.tgz -czf coverage-files.tgz coverage

Compress

• Took 0 min 0 sec . View more details on here
• Description: tar --exclude=system-tests-linux-files.tgz -czf system-tests-linux-files.tgz system-tests

Test Sync

• Took 4 min 45 sec . View more details on here
• Description: ./.ci/scripts/sync.sh

[codecov-io]

Codecov Report

Merging #4694 (fbb624c) into master (e232606) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #4694   +/-   ##
=======================================
  Coverage   76.24%   76.24%           
=======================================
  Files         163      163           
  Lines        9914     9914           
=======================================
  Hits         7559     7559           
  Misses       2355     2355           

Impacted Files	Coverage Δ
...ack/apm-server/aggregation/txmetrics/aggregator.go	93.36% <0.00%> (ø)