Add debug logs to Mint.HTTP2

[coveralls]

Pull Request Test Coverage Report for Build 67e6569946fc2ecf60ce2ba2bf0c588af37c2f35-PR-390

• 12 of 20 (60.0%) changed or added relevant lines in 5 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage decreased (-0.4%) to 86.328%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
lib/mint/http.ex	0	1	0.0%
lib/mint/http2.ex	9	11	81.82%
lib/mint/unsafe_proxy.ex	0	2	0.0%
lib/mint/http1.ex	2	5	40.0%
<!--	Total:	12	20	60.0%	-->

Totals
Change from base Build ca4e975214bfbb7627f6b52b3a5ea2a6b97964df:	-0.4%
Covered Lines:	1225
Relevant Lines:	1419

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💛 - Coveralls

[ericmj]

I think this should be an opt-in config. Enabling debug logging in an application can be very noisy if we log every received frame by default.

[whatyouhide]

I changed my mind about that after all the logger work in Erlang. Now it's so easy to filter out logs, for example, that I think it might be even easier to just log and have folks use logger filters to weed out messages they don't want?

[ericmj]

The logger filtering is opt out though right? So if you have a bunch of libraries logging you need to do a lot of work to only get your application logs.

[whatyouhide]

@ericmj yes, it's opt-out, which is why I was leaning on sticking mostly with debug logs.

What were you thinking as opt in? A logs: boolean() option when starting Mint connections?