feat: warn on too many concurrent requests

[zamnuts]

Fixes googleapis/nodejs-common#558 🦕

• Warns just the same as node.js max listeners: https://github.com/nodejs/node/blob/v12.16.2/lib/events.js#L381-L394
• Configurable with TEENY_REQUEST_WARN_CONCURRENT_REQUESTS env var or using teenyRequest.setStatOptions

[codecov]

Codecov Report

Merging #165 into master will increase coverage by 8.93%.
The diff coverage is 98.63%.

@@            Coverage Diff             @@
##           master     #165      +/-   ##
==========================================
+ Coverage   74.87%   83.80%   +8.93%     
==========================================
  Files           2        3       +1     
  Lines         410      630     +220     
  Branches       36       49      +13     
==========================================
+ Hits          307      528     +221     
+ Misses        103      102       -1     

Impacted Files	Coverage Δ
src/index.ts	71.58% <85.71%> (+2.06%)	⬆️
src/TeenyStatistics.ts	100.00% <100.00%> (ø)

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[stephenplusplus]

LGTM, but maybe let one more reviewer take a glance.

[bcoe]

Just a couple nits, and one additional test that I think would be worthwhile.

[bcoe]

👍 ideally once there's a test that we emit a process.on('warning') event.

[zamnuts]

@bcoe your feedback was implemented, but there were some interface changes. can you take one more pass and confirm all OK?