Rename HTTPTextFormat to TextMapPropagator

[TomRoSystems]

Fixes #536

Changes

As per spec open-telemetry/opentelemetry-specification#793

Trivial change.

[codecov]

Codecov Report

Merging #586 (d39a761) into main (4be5a94) will decrease coverage by 0.01%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main     #586      +/-   ##
==========================================
- Coverage   94.41%   94.40%   -0.02%     
==========================================
  Files         200      200              
  Lines        9068     9068              
==========================================
- Hits         8562     8561       -1     
- Misses        506      507       +1     

Impacted Files	Coverage Δ
...de/opentelemetry/trace/propagation/b3_propagator.h	97.95% <ø> (ø)
...entelemetry/trace/propagation/http_trace_context.h	86.79% <ø> (ø)
api/test/trace/propagation/b3_propagation_test.cc	100.00% <ø> (ø)
...pi/test/trace/propagation/http_text_format_test.cc	98.30% <100.00%> (ø)
sdk/test/metrics/counter_aggregator_test.cc	98.21% <0.00%> (-1.79%)	⬇️
sdk/src/logs/batch_log_processor.cc	93.82% <0.00%> (-1.24%)	⬇️
sdk/test/common/circular_buffer_test.cc	100.00% <0.00%> (+1.04%)	⬆️

[lalitb]

Thanks for the fix.