Add ErrorHandlerFunc to simplify creating ErrorHandlers

[yvrhdn]

This PR adds a convenience adapter to simplify setting the error handler.

Code before:

func main() {
	otel.SetErrorHandler(myErrorHandler{})

	// ...
}

type myErrorHandler struct {}

func (h myErrorHandler) Handle(err error) {
	// your custom error handling
	level.Error(log.Logger).Log("msg", "OpenTelemetry.ErrorHandler", "err", err)
}

After:

func main() {
	otel.SetErrorHandler(otel.ErrorHandlerFunc(func(err error) {
		// your custom error handling
		level.Error(log.Logger).Log("msg", "OpenTelemetry.ErrorHandler", "err", err)
	}))

	// ...
}

[codecov]

Codecov Report

Merging #2149 (4790a33) into main (23cb939) will increase coverage by 0.0%.
The diff coverage is 0.0%.

@@          Coverage Diff          @@
##            main   #2149   +/-   ##
=====================================
  Coverage   72.2%   72.2%           
=====================================
  Files        169     170    +1     
  Lines      12074   12076    +2     
=====================================
+ Hits        8724    8727    +3     
  Misses      3109    3109           
+ Partials     241     240    -1     

Impacted Files	Coverage Δ
error_handler.go	0.0% <0.0%> (ø)
...s/otlp/otlptrace/internal/connection/connection.go	16.4% <0.0%> (+1.5%)	⬆️

[MrAlias]

Outside of minor doc changes, I think these changes are a good addition.

[yvrhdn]

Thanks for the suggestion @MrAlias!