feat: pass agent options when using agent pool config

[zamnuts]

1. Defines the pool as used in @google-cloud/common: https://github.com/googleapis/nodejs-common/blob/v3.0.0/src/util.ts#L37-L40
2. All properties optionally defined in pool are passed through to the underlying agent, only if the agent is to be created
3. pool is now a "protected" member, only to make test cases possible given the singleton~ish variable bleed
4. fixed forever tests that were supposed to test https, but actually were testing http
5. enhanced existing agent tests to ensure the proxy data was actually picked up by the agent

• Make sure to open an issue as a bug/issue before writing your code! That way we can discuss the change, evaluate designs, and agree on the general idea
• Ensure the tests and linter pass
• Code coverage does not decrease (if any source code was changed)
• Appropriate docs were updated (if necessary)

Fixes #148 🦕

[codecov]

Codecov Report

Merging #149 into master will increase coverage by 0.47%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #149      +/-   ##
==========================================
+ Coverage   73.48%   73.95%   +0.47%     
==========================================
  Files           2        2              
  Lines         396      407      +11     
  Branches       41       40       -1     
==========================================
+ Hits          291      301      +10     
  Misses        104      104              
- Partials        1        2       +1     

Impacted Files	Coverage Δ
src/agents.ts	98.59% <100.00%> (-1.41%)	⬇️
src/index.ts	68.75% <100.00%> (+0.18%)	⬆️

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[bcoe]

This looks good to me, if @JustinBeckwith's comments about deferred loading are addressed.

Have you tested manually end to end (since we unfortunately don't have a system test suite).

[zamnuts]

@bcoe i did not test manually end-to-end (e.g. npm link ...). the unit tests have nock for which all tests passed FYI, but I'll go ahead and run system-test in a few libraries that use it and report back.

[zamnuts]

Manual regression testing was performed against nodejs-logging and nodejs-bigquery using their system-test with npm link teeny-request, and both were successful. I tried nodejs-pubsub, but its system-test didn't complete successfully (even w/o link, i.e. unrelated).

Note in the expandable log output below, the "using linked agent with options" output is not in this PR, but was a console logging entry used for illustrative purposes during testing. Additionally, some unnecessary information was redacted.

Evidence for nodejs-logging

nodejs-logging (master ✔) ᐅ npm link teeny-request
/home/user/.../nodejs-logging/node_modules/teeny-request -> /home/user/.nvm/versions/node/v10.19.0/lib/node_modules/teeny-request -> /home/user/.../teeny-request
nodejs-logging (master ✔) ᐅ git rev-parse HEAD
ce29b498ebb357403c093053d1b9989f1a56f5af
nodejs-logging (master ✔) ᐅ GOOGLE_APPLICATION_CREDENTIALS=/home/user/....json npm run system-test
> @google-cloud/logging@7.3.0 system-test /home/user/.../nodejs-logging
> mocha build/system-test --timeout 600000

  typescript consumer tests
using linked agent with options:  { maxSockets: Infinity }
using linked agent with options:  { maxSockets: Infinity }
using linked agent with options:  { maxSockets: Infinity }
    ✓ should have correct type signature for typescript users (7065ms)
    ✓ should have correct type signature for javascript users (8996ms)

  sinks
using linked agent with options:  { maxSockets: Infinity }
    ✓ should create a sink with a Bucket destination (1235ms)
using linked agent with options:  { maxSockets: Infinity }
using linked agent with options:  { maxSockets: Infinity }
    ✓ should create a sink with a Dataset destination (1404ms)
    ✓ should create a sink with a Topic destination (1828ms)
    metadata
      ✓ should set metadata (389ms)

...

Deleting 24 test logs...
23/24 logs deleted

  34 passing (3m)

Evidence for nodejs-bigquery

nodejs-bigquery (insert-retries ✔) ᐅ npm link teeny-request
/home/user/.../nodejs-bigquery/node_modules/teeny-request -> /home/user/.nvm/versions/node/v10.19.0/lib/node_modules/teeny-request -> /home/user/.../teeny-request
nodejs-bigquery (insert-retries ✔) ᐅ git rev-parse HEAD
ef1b538176103dea09c82e3ce4e2fe437a26c183
nodejs-bigquery (insert-retries ✔) ᐅ GOOGLE_APPLICATION_CREDENTIALS=/home/user/....json npm run system-test
> @google-cloud/bigquery@4.7.0 system-test /home/user/.../nodejs-bigquery
> mocha build/system-test --timeout 600000

  BigQuery
using linked agent with options:  { maxSockets: Infinity }
using linked agent with options:  { maxSockets: Infinity }
using linked agent with options:  { maxSockets: Infinity }
using linked agent with options:  { maxSockets: Infinity }
using linked agent with options:  { maxSockets: Infinity }
using linked agent with options:  { maxSockets: Infinity }
    ✓ should get a list of datasets (492ms)
using linked agent with options:  { maxSockets: Infinity }
    ✓ should allow limiting API calls (520ms)
using linked agent with options:  { maxSockets: Infinity }
    ✓ should return a promise (209ms)
using linked agent with options:  { maxSockets: Infinity }
    ✓ should allow limiting API calls via promises (518ms)

...

    ✓ should be able to use the d.ts (12252ms)


  95 passing (4m)

[JustinBeckwith]

LGTM assuming it works everywhere we expect it to 🙃