chore: add fields operation to TextMapPropagator

[dyladan]

Fixes #1485

[codecov]

Codecov Report

Merging #1615 into master will increase coverage by 0.02%.
The diff coverage is 94.73%.

@@            Coverage Diff             @@
##           master    #1615      +/-   ##
==========================================
+ Coverage   91.19%   91.22%   +0.02%     
==========================================
  Files         165      161       -4     
  Lines        5034     4727     -307     
  Branches     1026      963      -63     
==========================================
- Hits         4591     4312     -279     
+ Misses        443      415      -28     

Impacted Files	Coverage Δ
...y-api/src/context/propagation/TextMapPropagator.ts	14.28% <ø> (ø)
...i/src/context/propagation/NoopTextMapPropagator.ts	75.00% <50.00%> (-8.34%)	⬇️
...y-core/src/context/propagation/HttpTraceContext.ts	100.00% <100.00%> (ø)
...elemetry-core/src/context/propagation/composite.ts	100.00% <100.00%> (ø)
...tion-context/propagation/HttpCorrelationContext.ts	98.11% <100.00%> (+0.07%)	⬆️
...entelemetry-propagator-b3/src/B3MultiPropagator.ts	100.00% <100.00%> (ø)
...es/opentelemetry-propagator-b3/src/B3Propagator.ts	100.00% <100.00%> (ø)
...ntelemetry-propagator-b3/src/B3SinglePropagator.ts	100.00% <100.00%> (ø)
packages/opentelemetry-plugin-grpc/src/grpc.ts
packages/opentelemetry-plugin-grpc/src/utils.ts
... and 2 more