Convert id to string using strconv.Itoa
Convert id to string using strconv.Itoa

See https://github.com/golang/go/issues/32479

Fix #257.

Signed-off-by: Robert-André Mauchin <aa2d2b4191fcc92df7813016d4b511c37a1b9a82@gmail.com>

lightstep/lightstep-tracer-go,lightstep/lightstep-tracer-go