feat: structured logs with uber/zap

[iwpnd]

Hi 👋

As discussed in #831 I added the option to use structured logs with uber/zap while falling back to the currently implemented default if --logger is not specifically used. Log levels conform to the current implementation with the exception of log level TRACE that is not supported by zap. I use Debug as a fall back here.

Looking forward for your feedback

[coveralls]

Pull Request Test Coverage Report for Build 3f60b86ad-PR-848

• 7 of 73 (9.59%) changed or added relevant lines in 3 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage decreased (-0.2%) to 45.32%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
internal/log/log.go	5	10	50.0%
cmd/tegola/cmd/root.go	2	18	11.11%
internal/log/zap.go	0	45	0.0%
<!--	Total:	7	73	9.59%	-->

Totals
Change from base Build 7ac37b942:	-0.2%
Covered Lines:	5583
Relevant Lines:	12319

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💛 - Coveralls

[iwpnd]

The main thing I want to think through here is, do we really want to support 2 types of loggers?

While zap is fast, text logs have the edge in terms of performance because json serialization has an overhead. Giving the user the option is only fair as not every users wants to reap the benefits of json logs, or doesn't even care at all.

But ultimately that's your decision to make of course. :)

[iwpnd]

Do you want to move forward with this or drop @ARolek @gdey ?

[ARolek]

@iwpnd we should move forward with it! I know others have asked about this as well. Let me bump @gdey so he can provide some input as well.