PSA: tracing is now controlled by cluster settings

I just merged #16265 which removes the COCKROACH_ENABLE_TRACING and COCKROACH_LIGHTSTEP_TOKEN env variables. We can now control tracing on a live cluster via cluster settings.

To enable traces in the /debug endpoint:
SET CLUSTER SETTING trace.debug.enable = true;

To enable lightstep traces:
SET CLUSTER SETTING trace.lightstep.token = ‘…’;

To disable lightstep, set an empty token.