Add support for adding callables as extra tags and some major improvements

[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

handler = logging_loki.LokiQueueHandler(
    Queue(-1),
    url="https://my-loki-instance/loki/api/v1/push", 
    tags={"application": "my-app", 'span_id': add_span_id, 'trace_id': add_trace_id},
    auth=("username", "password"),
    version="1"
)

As an added extra, I've added submission of extra logging data, in order to enable more structured logging in the system. The extra data, such as tags, will be attached to

"values": [
    [ "<unix epoch in nanoseconds>", "<log line>", {"trace_id": "0242ac120002", "user_id": "superUser123"}]
]

Additionally, empty tags will be discarded. Previously you had positions like exc_text=None. They won't be sent now.

Please note that labels will be sent only using the V1 Loki protocol.