fix(statsd) deal with nil ctx.api on 404

[chyh1990]

Summary

Fix statsd plugin, which crashes on 404 requests with:

2017/08/02 11:55:05 [error] 109#0: *46932 lua entry thread aborted: runtime error: /usr/local/share/lua/5.1/kong/plugins/statsd/handler.lua:63: attempt to index field 'api' (a nil value)
stack traceback:
coroutine 0:
	/usr/local/share/lua/5.1/kong/plugins/statsd/handler.lua: in function </usr/local/share/lua/5.1/kong/plugins/statsd/handler.lua:54>, context: ngx.timer, client: 10.20.1.1, server: 0.0.0.0:8000
2017/08/02 11:55:05 [error] 109#0: *46936 lua entry thread aborted: runtime error: /usr/local/share/lua/5.1/kong/plugins/statsd/handler.lua:63: attempt to index field 'api' (a nil value)

Full changelog

• dereference message.api only if it is not nil

[p0pr0ck5]

I think this should follow the pattern presented in #2702

[thibaultcha]

@chyh1990 Any updates on your side? I do agree with @p0pr0ck5 that this should follow the behavior of the recent datadog fix.

[p0pr0ck5]

@chyh1990 ping? ^

[cyrilgdn]

Any news? Can we help on this? We have the same problem
BTW @p0pr0ck5 , I agree that all stats plugins should have the same behavior but it can be interesting to have stats on non-api logs. (I want to be alerted if there are too many of these queries for example)

[thibaultcha]

@cyrilgdn The author seems to be unresponsive (and we are sorry for the delay on our side). Feel free to propose a change of your own following @p0pr0ck5's recommendation.

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