Rename message events to events

[jtescher]

Message events were a holdover from OpenCensus I believe.

[codecov]

Codecov Report

Merging #530 (fd9464c) into main (4e7613b) will decrease coverage by 0.0%.
The diff coverage is 93.1%.

@@           Coverage Diff           @@
##            main    #530     +/-   ##
=======================================
- Coverage   52.2%   52.2%   -0.1%     
=======================================
  Files         96      96             
  Lines       8514    8513      -1     
=======================================
- Hits        4452    4451      -1     
  Misses      4062    4062             

Impacted Files	Coverage Δ
opentelemetry-jaeger/src/exporter/mod.rs	47.8% <0.0%> (ø)
opentelemetry/src/sdk/trace/span.rs	95.0% <90.9%> (-0.1%)	⬇️
opentelemetry-datadog/src/exporter/model/mod.rs	63.6% <100.0%> (ø)
opentelemetry-zipkin/src/exporter/model/mod.rs	81.6% <100.0%> (ø)
opentelemetry-zipkin/src/exporter/model/span.rs	94.1% <100.0%> (ø)
opentelemetry/src/sdk/export/trace/mod.rs	98.1% <100.0%> (ø)
opentelemetry/src/sdk/trace/tracer.rs	85.8% <100.0%> (ø)
opentelemetry/src/testing/trace.rs	63.4% <100.0%> (ø)
opentelemetry/src/trace/tracer.rs	63.2% <100.0%> (ø)

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