Refactor metrics logger

[dtaniwaki]

I refactored the metrics logger a little bit. It can be the start point of issue #145, or this change may be enough.

[dtaniwaki]

Looks like https://github.com/go-kit/kit/tree/master/log doesn't support async logging, which we will surely need.

@ordovicia @everpeace So for the time being, what do you think this refactoring? Do you think merging file metrics logger and stdout metrics loggers is not intuitive?

[ordovicia]

We can now remove stdout_writer.go.

For now, merging the file writer and stdout writer sounds good to me.
For a future possibility, we can modify the writer to take io.Writer interface instead of a concrete type os.File.

[ordovicia]

IMHO it is not good to implicitly set the destination to stdout when path is empty.
I prefer to explicitly specify /dev/stdout.

[ordovicia]

It is not necessarily a good idea, but we could accept stdout as well as /dev/stdout (and stderr) for convenience as the output destination.
If we take this way, the name Path should be changed, since stdout is not a path.

[dtaniwaki]

@ordovicia I updated the code. Could you check it again when you have time?

For a future possibility, we can modify the writer to take io.Writer interface instead of a concrete type os.File.

I agree with this idea, and I tried to implement it, however, I wasn't able to find a good and simple solution to avoid the file descriptor leak. I think we can make it in another PR if necessary.

[ordovicia]

standard out if not specfiied

Could you please update this comment?

After this line is fixed, I will merge this PR. Thank you!

[ordovicia]

Thank you.
The last some commits are by me. I believe they didn't introduce regression.