[feature request] allow export instrumentationName

[futurist]

A clear and concise description of what you want to happen.

currently it's not possible to get tracerProvider from otel, like below:

tracer := otel.GetTracerProvider().Tracer(instrumentationName)

since below is not exported:

const (
	instrumentationName = "github.com/hertz-contrib/obs-opentelemetry"
)

Describe alternatives you've considered

Make instrumentationName be exported so can get the same tracer of hertz, then add new span into exiting hertz tracer.

[CoderPoet]

This is the tracer of the internal library, and it is recommended that the tracer of the business layer be separated from it. Consider customizing a tracer name, like:

otel.GetTracerProvider().Tracer("your business tracer")