inject method
Implementation
@override
void inject(api.Context context, dynamic carrier, api.TextMapSetter setter) {
final spanContext = context.spanContext;
setter
..set(
carrier,
_traceParentHeaderKey,
'$_traceVersion-${spanContext!.traceId.toString()}-'
'${spanContext.spanId.toString()}-'
'${spanContext.traceFlags.toRadixString(16).padLeft(2, '0')}')
..set(carrier, _traceStateHeaderKey, spanContext.traceState.toString());
}