Dummy callbacks. It can be useful to reuse some of these functions in a real subscriber that doesn't want to handle all events, but only some of them.
val on_init : 'a->time_ns:'b-> unit
val on_shutdown : 'a->time_ns:'b-> unit
val on_name_thread : 'a->time_ns:'b->tid:'c->name:'d-> unit
val on_name_process : 'a->time_ns:'b->tid:'c->name:'d-> unit
val on_message : 'a->time_ns:'b->tid:'c->span:'d->data:'e->'f-> unit
val on_counter : 'a->time_ns:'b->tid:'c->data:'d->name:'e->'f-> unit
val on_enter_span :
'a->__FUNCTION__:'b->__FILE__:'c->__LINE__:'d->time_ns:'e->tid:'f->data:'g->name:'h->'i->
unit
val on_exit_span : 'a->time_ns:'b->tid:'c->'d-> unit
val on_add_data : 'a->data:'b->'c-> unit
val on_enter_manual_span :
'a->__FUNCTION__:'b->__FILE__:'c->__LINE__:'d->time_ns:'e->tid:'f->parent:'g->data:'h->name:'i->flavor:'j->trace_id:'k->'l->
unit
val on_exit_manual_span :
'a->time_ns:'b->tid:'c->name:'d->data:'e->flavor:'f->trace_id:'g->'h->
unit
val on_extension_event : 'a->time_ns:'b->tid:'c->'d-> unit