Add support for adding callables as extra fields

[piotrmaslanka]

Support was added for specifying callables as extra fields. This is super useful if we're trying to attach span_id and trace_id to a log entry in order to cross-match it with a tracing system, such as Jaeger, and we need that data dynamically rather than statically at startup.

Example:

get_context = lambda: tracer.active_span.context
add_trace_id = lambda: hex(get_context().trace_id)[2:] if tracer is not None and tracer.active_span is not None else None
add_span_id = lambda: hex(get_context().span_id)[2:] if tracer is not None and tracer.active_span else None

formatter = LogstashFormatter(extra={'span_id': add_span_id, 'trace_id': add_trace_id})
logging.getLogger().setFormatter(formatter)

[eht16]

I wonder if we should catch errors from the callable (and if so, move it into a seperate method).

If the callable throws an error, this way the whole log event will be lost.
If the error is handled, the log event can be sent anyways and we could log the error for later analysis.

What do you think?

[eht16]

@piotrmaslanka what do you think?