tracing: Including all current data in `tracestate` breaks character limit

[lobsterkatie]

This and #425 are a continuation of #410, to make sure points I brought up there don't get lost now that it's closed.

With the addition of user data and transaction name to tracestate, we are now reliably over the character limit (256) listed in the spec.

For reference:

Tracestate data:

{
    "trace_id": "12312012123120121231201212312012",
    "environment": "dogpark",
    "release": "off.leash.park",
    "public_key": "dogsarebadatkeepingsecrets",
    "user": {"id": 12312013, "segment": "bigs"},
    "transaction": "/interactions/other-dogs/new-dog/"
}

tracestate with without either: 196 characters
tracestate with user data: 256 characters
tracestate with transaction name: 264 characters
tracestate with both: 324 characters