expose logger

[nicolasparada]

Expose the logger from fluent-bit so we can write logs using the same format and respect the same level configured in fluent-bit.

@cosmo0920 can you tell me all the levels that fluent-bit has in its logger and whether it is an structured logger or not, so we can come with a nice golang interface.

[cosmo0920]

I tested to expose fluent-bit's logger interface into Golang world.
But, there is a few issues to handle in Golang side:

1. We wouldn't handle variable-argument lists at flb_log_print function in Golang caller:
   ref: https://github.com/fluent/fluent-bit/blob/master/src/flb_log.c#L326
   ref: cmd/cgo: cannot call functions with variable-argument lists. golang/go#975

   a. Or, variable-argument lists operation should be proceeded in Golang side with fmt.Sprintf or something equivalent and then passing processed string into exposed log interface?

2. Only retrieving fluent-bit's log level and a path of log file is better?

I'm not sure which is the better choice for our use case.

[cosmo0920]

I'm considering the following:

1. We can bind fluent-bit's logging interface with novar (no variable-argument) version.
2. We can proceed variable-argument with fmt.sprintf or its equivalents before passing binded fluent-bit's logger

[cosmo0920]

I created a PR to handle the first one:

We can bind fluent-bit's logging interface with novar (no variable-argument) version.

[nicolasparada]

Off course, the Golang function can use variadic arguments, and build the final string using fmt.Sprint and pass it to fluentbit.