Type of callbacks.

Create a Callback that optionally subscribes to one or more runtime events. The first int supplied to callbacks is the ring buffer index. Each domain owns a single ring buffer for the duration of the domain's existence. After a domain terminates, a newly spawned domain may take ownership of the ring buffer. A runtime_begin callback is called when the runtime enters a new phase (e.g a runtime_begin with EV_MINOR is called at the start of a minor GC). A runtime_end callback is called when the runtime leaves a certain phase. The runtime_counter callback is called when a counter is emitted by the runtime. lifecycle callbacks are called when the ring undergoes a change in lifecycle and a consumer may need to respond. alloc callbacks are currently only called on the instrumented runtime. lost_events callbacks are called if the consumer code detects some unconsumed events have been overwritten.

add_user_event ty callback t extends t to additionally subscribe to user events of type ty. When such an event happens, callback is called with the corresponding event and payload.