refactor: normalize namespace import name for @opentelemetry/api

[legendecas]

Which problem is this PR solving?

• Address comment on fix: observers should not expose bind/unbind method #1001 (comment)

Short description of the changes

• Renamed all namespace import of @opentelemetry/api variable name to api.

[codecov-io]

Codecov Report

Merging #1016 into master will not change coverage.
The diff coverage is 97.14%.

@@           Coverage Diff           @@
##           master    #1016   +/-   ##
=======================================
  Coverage   95.00%   95.00%           
=======================================
  Files         212      212           
  Lines        8806     8806           
  Branches      796      796           
=======================================
  Hits         8366     8366           
  Misses        440      440           

Impacted Files	Coverage Δ
...ackages/opentelemetry-exporter-jaeger/src/types.ts	100.00% <ø> (ø)
...ackages/opentelemetry-exporter-zipkin/src/types.ts	100.00% <ø> (ø)
...ckages/opentelemetry-exporter-zipkin/src/zipkin.ts	98.33% <ø> (ø)
...ackages/opentelemetry-metrics/test/Batcher.test.ts	92.59% <ø> (ø)
...lugin-https/test/functionals/https-disable.test.ts	100.00% <ø> (ø)
...kages/opentelemetry-metrics/src/BoundInstrument.ts	92.85% <83.33%> (ø)
packages/opentelemetry-metrics/src/Meter.ts	96.29% <87.50%> (ø)
packages/opentelemetry-core/src/common/time.ts	95.38% <100.00%> (ø)
...ackages/opentelemetry-core/src/trace/TraceState.ts	100.00% <100.00%> (ø)
...ckages/opentelemetry-core/test/common/time.test.ts	100.00% <100.00%> (ø)
... and 10 more

[mayurkale22]

Nice!

[obecny]

lgtm