Add TM GC CPU fraction to stat endpoint

[rob05c]

What does this PR do?

Adds TM GC CPU fraction to stat endpoint, for debugging and profiling.

Which TC components are affected by this PR?

• Documentation
• Grove
• Traffic Analytics
• Traffic Monitor
• Traffic Ops
• Traffic Ops ORT
• Traffic Portal
• Traffic Router
• Traffic Stats
• Traffic Vault
• Other _________

What is the best way to verify this PR?

Run Monitor, verify /publish/Stats endpoint has gc-cpu-fraction key.

Check all that apply

• This PR includes tests
• This PR includes documentation updates
• This PR includes an update to CHANGELOG.md
• This PR includes all required license headers
• This PR includes a database migration (ensure that migration sequence is correct)
• This PR fixes a serious security flaw. Read more: www.apache.org/security

[asfgit]

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Test PASSed.

[ezelkow1]

Looks good, GCCPUfraction has been around since go1.5 and could be helpful to monitor usage

[dneuman64]

I know @ezelkow1 took a look at the code changes here, but I think this PR would benefit from a test or two.
It looks like there currently isn't any tests for this Stats struct (if there is, why didn't get get updated?), so I think our code would benefit from at least a test of the struct.

IMO Traffic Monitor is light on tests as is, and this seems like a good time to add some. It would definitely help future developers.

[asfgit]

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Test FAILed.

[asfgit]

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Test FAILed.

[dneuman64]

Thanks Rob! I know it can be a PITA do to tests, and it's frustrating when you have to write a test after changing one little thing. I am definitely not "unit test guy" but I think even a little test like this can make our products better and make the lives of future developers much easier. I appreciate you writing the test.