Add message sizes to addMessageEvent function

[draffensperger]

Message size parameters were added in #323 and this now adds them to the addMessageEvent function on span as optional parameters.

This also adds a comment to the id field of the MessageEvent interface to explain that it is a hexadecimal string that should increment as new messages are sent.

cc/ @odeke-em

[codecov-io]

Codecov Report

Merging #344 into master will decrease coverage by 0.02%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #344      +/-   ##
==========================================
- Coverage   95.17%   95.14%   -0.03%     
==========================================
  Files         122      122              
  Lines        8472     8474       +2     
  Branches      750      750              
==========================================
  Hits         8063     8063              
- Misses        409      411       +2

Impacted Files	Coverage Δ
src/stackdriver-monitoring.ts	81.05% <0%> (-2.11%)	⬇️
test/test-span.ts	100% <0%> (ø)	⬆️
src/trace/model/types.ts	100% <0%> (ø)	⬆️
src/trace/model/span-base.ts	98.92% <0%> (+0.01%)	⬆️

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[mayurkale22]

LGTM, thanks for doing this. We need to export these fields in the Stackdriver exporter (https://github.com/census-instrumentation/opencensus-node/blob/master/packages/opencensus-exporter-stackdriver/src/stackdriver-cloudtrace-utils.ts#L84), might be in new PR.

[odeke-em]

Cool, thank you @draffensperger for the follow-up and for adding them here too!

[draffensperger]

Yes, sure, I can add the exporter logic in a new PR. Probably need it for Stackdriver and agent.