fix: force http and https clients to be patched

[kjin]

Requests from node-fetch currently might not get traced, because we use it in the Trace Agent (for authentication). Because it's loaded in the Trace Agent before we set up monkeypatching, we don't have a way to hook into its require of http or https. Therefore, for http, if it's the only module to be included that would require http, then http will never get patched, and HTTP requests will never get traced. The same is independently true of https. (Common HTTP frameworks will typically require http, alleviating this problem for HTTP requests. However, in a Cloud Functions environment, this might not apply, as even the HTTP framework is required before the Trace Agent has a chance to set up monkeypatching.)

We address this simply by requiring http and https when the Trace Agent starts.

Fixes #1081

[ofrobots]

I see the change, and I see that it fixes an issue with tracing of node-fetch, however, I don't see an explanation for why existing code wasn't working and why the change here fixes it. Can you include background on what the bug was and elaborate on why force-loading http and https fixes it?

[codecov]

Codecov Report

Merging #1084 into master will increase coverage by 0.03%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #1084      +/-   ##
==========================================
+ Coverage   94.93%   94.96%   +0.03%     
==========================================
  Files          96       98       +2     
  Lines        6397     6441      +44     
  Branches      498      499       +1     
==========================================
+ Hits         6073     6117      +44     
  Misses        166      166              
  Partials      158      158

Impacted Files	Coverage Δ
test/plugins/test-trace-http.ts	100% <100%> (ø)	⬆️
test/plugins/test-trace-node-fetch.ts	100% <100%> (ø)
test/web-frameworks/express-secure.ts	100% <100%> (ø)
src/tracing.ts	90.32% <100%> (+0.32%)	⬆️

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