There's already a constant for it: TRACER_IP_TAG_KEY.

Jaeger uses ip tag to group spans around individual machines for clock skew adjustment and with futures it's easy to mess up and make child spans outlive parents, triggering time travel bamboozle:

• Warn about time adjustment in tags jaegertracing/jaeger#2052

It's probably best to infer ip tag automatically just like C++ and Go clients do.