class TraceHandler

Called by the trace engine to ask whether the specified category is enabled.

This method may be called frequently so it must be efficiently implemented.

Clients may cache the results while a trace is running; dynamic changes

to the enabled categories may go unnoticed until the next trace.

|category| is the name of the category.

Called by instrumentation on any thread. Must be thread-safe.

Called by the trace engine to indicate it has completed startup.

Called by the trace engine when tracing has stopped.

The trace collection status is |ZX_OK| if trace collection was successful.

An error indicates that the trace data may be inaccurate or incomplete.

|disposition| is |ZX_OK| if tracing stopped normally, otherwise indicates

that tracing was aborted due to an error. If records were dropped (due

to the trace buffer being full) then |disposition| is |ZX_ERR_NO_MEMORY|.

Called on an asynchronous dispatch thread.

Called by the trace engine when tracing has terminated.

Called by the trace engine in streaming mode to indicate a buffer is full.

This is only used in streaming mode where double-buffering is used.

|wrapped_count| is the number of times writing to the buffer has

switched from one buffer to the other.

|durable_buffer_offset| is the offset into the durable buffer when the

buffer filled. It is provided so that TraceManager can save the data

thus far written to the durable buffer.

Called by the trace engine to send an alert.