[BUG]: panic if `http.DefaultTransport` is replaced

[austindrenski]

Runtime panic in v1.55.1 if http.DefaultTransport has been replaced, following the changes in #641:

aws-lambda-go/lambda/runtime_api_client.go

Line 46 in 94b293d

transport := http.DefaultTransport.(*http.Transport).Clone()

e.g.,

import (
	"net/http"
	"net/http/httptrace"

	"go.opentelemetry.io/contrib/instrumentation/net/http/httptrace/otelhttptrace"
	"go.opentelemetry.io/contrib/instrumentation/net/http/otelhttp"
)

// ...

http.DefaultTransport = otelhttp.NewTransport(http.DefaultTransport, otelhttp.WithClientTrace(func(ctx context.Context) *httptrace.ClientTrace {
	return otelhttptrace.NewClientTrace(ctx)
}))

https://github.com/open-telemetry/opentelemetry-go-contrib/blob/ce138eb092f223e4e41e0420c58fc34910bc2b56/instrumentation/net/http/otelhttp/transport.go#L44-L68

See: #641

/cc @benrkia