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

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 82.1%. Comparing base (2b554ec) to head (d255c02).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@          Coverage Diff          @@
##            main   #8153   +/-   ##
=====================================
  Coverage   82.1%   82.1%           
=====================================
  Files        195     195           
  Lines      13680   13680           
=====================================
+ Hits       11243   11244    +1     
+ Misses      2031    2029    -2     
- Partials     406     407    +1     

Files with missing lines	Coverage Δ
instrumentation/net/http/otelhttp/config.go	83.0% <100.0%> (+1.5%)	⬆️

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[flc1125]

@sonalgaud12 Please fix the lint; we can use make ci for local verification.

[dmathieu]

Could you solve the lint issue please?

[sonalgaud12]

Hey @flc1125 , Can you please review this?