use structlog's get_logger if it is available

[beniwohli]

If the user has set up structlog, this will add some additional context
to each log line, like level and timestamp. E.g.

{"event": "Read traceparent header 00-42d5718b9ac8289e439a688afb17cbae-d6f9a1dad4c90dec-01"}

becomes

{"event": "Read traceparent header 00-42d5718b9ac8289e439a688afb17cbae-d6f9a1dad4c90dec-01",
"timestamp": "2019-09-17T15:37:00.049627Z", "logger": "elasticapm.traces", "level": "debug"}

This can be overridden by setting an environment variable
ELASTIC_APM_DISABLE_STRUCTLOG to "true".

[basepi]

This seems reasonable to me. Definitely needs to be documented, though.

Is there any way we can discover whether they are actually using structlog, as opposed to just have it installed? Having this on by default is definitely the user-friendly behavior (if they have it installed, they're probably using it) but having it off by default is probably the "safer" option.

[basepi]

I'm seriously considering flip-flopping on this again (after all my work on the log correlation testing).

On the one hand, off by default is safer. But if they have structlog installed it's not like they'll be surprised to see different style log messages from us. And if I'm doing structured logging it would sure be convenient for the apm agent to switch automatically.

Basically, what I'm trying to say is I am fine either way on this, but honestly I'm leaning toward your first implementation, on by default. ¯_(ツ)_/¯

[beniwohli]

@basepi as a compromise, we could check structlog.is_configured(), but I'm not a terribly huge fan of that approach, because then the behavior becomes dependent of when exactly elasticapm is imported first. It would only use structlog if it is configured before elasticapm is imported. Or even worse, some of the loggers could use structlog, while others wouldn't, depending on when exactly each module was first imported

[basepi]

Yeah that seems like the worst of both worlds. What's the quote? "A good compromise is when both parties are dissatisfied"? ;)