otelhttp: export Event type for WithMessageEvents

[sonalgaud12]

This PR fixes issue #8132

The WithMessageEvents option previously accepted an unexported type (event).
This change exports the type as Event.

[sonalgaud12]

Hello @dmathieu , OpenTelemetry Go enums usually prefix constants with the type name (e.g. SpanKindServer, SpanKindClient). So shall I change ReadEvents, WriteEvents to EventRead, EventWrite .

[codecov]

Codecov Report

❌ Patch coverage is 0% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 81.7%. Comparing base (8ef069f) to head (33a327d).

Files with missing lines	Patch %	Lines
instrumentation/net/http/otelhttp/config.go	0.0%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##            main   #8153     +/-   ##
=======================================
- Coverage   81.8%   81.7%   -0.1%     
=======================================
  Files        194     194             
  Lines      13362   13362             
=======================================
- Hits       10931   10920     -11     
- Misses      2033    2044     +11     
  Partials     398     398             

Files with missing lines	Coverage Δ
instrumentation/net/http/otelhttp/config.go	82.0% <0.0%> (ø)

... and 1 file with indirect coverage changes

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