feat(breaking!): support API endpoint override

[JustinBeckwith]

No description provided.

[nolanmar511]

There are two things I'll need to be careful about with respect to this change:

1. Updating @google-cloud/common has broken our backoff when multiple agents with the same deployment are running. We don't have automated tests for this, and I'll need to test this manually.

2. It looks like this change is a breaking change for our agent (since setting baseApiUrl no longer has an effect; I know our probers that run against our test environment will be broken by this). Since we recently bumped out major version (for dropping Node 6), merging this definitely requires careful consideration for us. I don't want to need to bump our major version too frequently.

[codecov]

Codecov Report

Merging #509 into master will decrease coverage by 0.19%.
The diff coverage is 75%.

@@            Coverage Diff            @@
##           master     #509     +/-   ##
=========================================
- Coverage   76.57%   76.38%   -0.2%     
=========================================
  Files           7        7             
  Lines         269      271      +2     
  Branches       56       56             
=========================================
+ Hits          206      207      +1     
- Misses         48       49      +1     
  Partials       15       15

Impacted Files	Coverage Δ
ts/src/config.ts	100% <ø> (ø)	⬆️
ts/src/profiler.ts	89.44% <100%> (+0.13%)	⬆️
ts/src/index.ts	58.69% <50%> (-1.09%)	⬇️

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[nolanmar511]

Added a "do not merge" label because I'd like to get a release of @google-cloud/profiler out before merging this change.

[bcoe]

this looks reasonable to me. @nolanmar511, let's try to figure out why release-please is not working on this repo, and release the next version of this library using automation.