feat: add keys operation to getter

[dyladan]

Fixes #1486

Changes getter and setter to be an interface rather than a function so more operations may be added later.

[codecov]

Codecov Report

Merging #1576 into master will decrease coverage by 0.22%.
The diff coverage is 74.19%.

@@            Coverage Diff             @@
##           master    #1576      +/-   ##
==========================================
- Coverage   91.40%   91.18%   -0.23%     
==========================================
  Files         165      164       -1     
  Lines        5013     5024      +11     
  Branches     1023     1026       +3     
==========================================
- Hits         4582     4581       -1     
- Misses        431      443      +12     

Impacted Files	Coverage Δ
...ackages/opentelemetry-shim-opentracing/src/shim.ts	87.70% <ø> (ø)
...y-api/src/context/propagation/TextMapPropagator.ts	14.28% <14.28%> (ø)
packages/opentelemetry-api/src/api/propagation.ts	56.25% <33.33%> (-1.33%)	⬇️
...telemetry-plugin-grpc-js/src/server/patchServer.ts	88.52% <50.00%> (-1.48%)	⬇️
packages/opentelemetry-plugin-grpc/src/grpc.ts	93.26% <75.00%> (-0.46%)	⬇️
packages/opentelemetry-api/src/api/global-utils.ts	100.00% <100.00%> (ø)
...i/src/context/propagation/NoopTextMapPropagator.ts	83.33% <100.00%> (ø)
...-core/src/context/propagation/B3MultiPropagator.ts	100.00% <100.00%> (ø)
...metry-core/src/context/propagation/B3Propagator.ts	100.00% <100.00%> (ø)
...core/src/context/propagation/B3SinglePropagator.ts	100.00% <100.00%> (ø)
... and 6 more

[vmarchaud]

should the interfaces lives in types file or maybe the Noop ?

[dyladan]

The interface was originally in its own file, but I think it makes sense to keep it more tightly coupled to the TextMapPropagator since this getter is specifically for this type of propagator. If other propagator types are added in the future like binary they may have different interfaces.

[obecny]

I'm fine with keeping the interfaces together with class if they are not exported or keeping them in some general file like types.ts. Otherwise I would rather keep them in separate file or in general types.ts as it quickly gets messy later with any refactoring or when trying to find the desired interface.
Because of that:

If other propagator types are added in the future like binary they may have different interfaces.

Can you change names:

• Setter into TextMapSetter
• Getter into TextMapGetter
• defaultGetter into defaultTextMapGetter

and then mentioned types.ts should become textMapTypes.ts ?
WDYT ?

[dyladan]

The TextMapPropagator is already just an interface. Won't it be weird to have a types file, but then also have some types (the TextMapPropagator interface) defined in a different file? I agree with the renaming though.

[obecny]

you right it is interface already I was blind :/

[dyladan]

I did the rename as I think that is a good idea to prevent future incompatibilities

[mwear]

LGTM

[obecny]

lgtm