fix(tracing): span processor should receive a readable span as parameters

[legendecas]

Which problem is this PR solving?

• Make SpanProcessor accepts ReadableSpan as parameters in accordance with spec.
• Fixes custom SpanProcessors api definition issue #1028

Short description of the changes

• Removed tracing.Span.toReadableSpan as tracing.Spans are already implements ReadableSpan.

[codecov-io]

Codecov Report

Merging #1037 into master will decrease coverage by 0.00%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #1037      +/-   ##
==========================================
- Coverage   95.13%   95.13%   -0.01%     
==========================================
  Files         212      212              
  Lines        8899     8895       -4     
  Branches      801      801              
==========================================
- Hits         8466     8462       -4     
  Misses        433      433              

Impacted Files	Coverage Δ
...entelemetry-exporter-zipkin/test/transform.test.ts	100.00% <ø> (ø)
packages/opentelemetry-tracing/src/Span.ts	100.00% <ø> (ø)
...lemetry-plugin-http/test/functionals/utils.test.ts	99.40% <100.00%> (ø)
...es/opentelemetry-tracing/src/MultiSpanProcessor.ts	100.00% <100.00%> (ø)
...ges/opentelemetry-tracing/src/NoopSpanProcessor.ts	60.00% <100.00%> (ø)
...telemetry-tracing/src/export/BatchSpanProcessor.ts	95.74% <100.00%> (ø)
...elemetry-tracing/src/export/SimpleSpanProcessor.ts	78.57% <100.00%> (ø)
...ntelemetry-tracing/test/MultiSpanProcessor.test.ts	93.10% <100.00%> (+1.10%)	⬆️
packages/opentelemetry-tracing/test/Span.test.ts	100.00% <100.00%> (ø)

[mayurkale22]

Sweet!