0.4

CHANGES:

• add ?data to counter_int and counter_float

• add float to user data

• add add_data_to_current_span and add_data_to_manual_span

• make explicit_span.meta mutable

• trace-tef: write to trace.json if env variable TRACE is either 1 or true

• trace-tef: emit function name, if provided, as a metadata key/value pair

• re-export trace.core in trace

• perf: in trace-tef, use broadcast instead of signal in the job queue