Rename get_tracer to tracer

[jtescher]

Rename tracer provider methods as it is less idiomatic for rust methods to have the get prefix.

[codecov]

Codecov Report

Merging #586 (05760e1) into main (068aacc) will not change coverage.
The diff coverage is 72.2%.

@@          Coverage Diff          @@
##            main    #586   +/-   ##
=====================================
  Coverage   54.7%   54.7%           
=====================================
  Files        102     102           
  Lines       8846    8846           
=====================================
  Hits        4842    4842           
  Misses      4004    4004           

Impacted Files	Coverage Δ
opentelemetry-datadog/src/exporter/mod.rs	12.2% <0.0%> (ø)
opentelemetry-jaeger/src/exporter/mod.rs	47.7% <0.0%> (ø)
opentelemetry-zipkin/src/exporter/mod.rs	0.0% <0.0%> (ø)
opentelemetry/src/sdk/export/trace/stdout.rs	9.6% <ø> (ø)
opentelemetry/src/sdk/propagation/composite.rs	95.6% <ø> (ø)
opentelemetry/src/trace/context.rs	37.1% <ø> (ø)
opentelemetry/src/trace/mod.rs	50.0% <ø> (ø)
opentelemetry/src/global/trace.rs	17.0% <71.4%> (ø)
opentelemetry/src/sdk/trace/provider.rs	81.8% <100.0%> (ø)
opentelemetry/src/sdk/trace/span.rs	95.0% <100.0%> (ø)
... and 2 more

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