Use enum for MessageEvent Type, Link Type and SpanKind

[mayurkale22]

Fixes #333

[draffensperger]

I do like the numeric enums here so on balance I recommend keeping them this way (nice correspondence with the protos too). But on the other hand, use of a string enum could enable the breaking change to be less significant since the strings 'SENT' and 'RECEIVED' would still be valid. Just mentioning it because it's in my mind, but recommend keeping it this way!

[mayurkale22]

I compared with other lang. implementation, we use numeric enums for these (Kind, Link and MessageEvent) types. I am trying to get most of the breaking change in this and next release before going to Beta.

[draffensperger]

Makes sense. The sooner the breaking changes come the less the pain in the future.

[codecov-io]

Codecov Report

Merging #334 into master will increase coverage by 0.04%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #334      +/-   ##
==========================================
+ Coverage   94.93%   94.98%   +0.04%     
==========================================
  Files         120      120              
  Lines        8315     8332      +17     
  Branches      739      741       +2     
==========================================
+ Hits         7894     7914      +20     
+ Misses        421      418       -3

Impacted Files	Coverage Δ
src/instana.ts	75.86% <0%> (-0.41%)	⬇️
test/test-grpc.ts	99.58% <0%> (ø)	⬆️
test/test-root-span.ts	100% <0%> (ø)	⬆️
test/test-exporter-buffer.ts	100% <0%> (ø)	⬆️
test/test-ocagent.ts	91.74% <0%> (ø)	⬆️
test/test-console-exporter.ts	100% <0%> (ø)	⬆️
test/test-tracer.ts	100% <0%> (ø)	⬆️
test/test-instana.ts	100% <0%> (ø)	⬆️
src/trace/model/span-base.ts	98.91% <0%> (ø)	⬆️
test/test-span.ts	100% <0%> (ø)	⬆️
... and 7 more

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