Use Monotonic time for calculating request latency in Plug.Logger

[mje113]

According to http://erlang.org/doc/apps/erts/time_correction.html#Erlang_Monotonic_Time to Measure Elapsed Time:

Take timestamps with erlang:monotonic_time/0 and calculate the time difference using ordinary subtraction. The result will be in native time unit. If you want to convert the result to another time unit, you can use erlang:convert_time_unit/3.

An easier way to do this is to use erlang:monotonic_time/1 with the desired time unit. However, you can then lose accuracy and precision.

[mje113]

I just realized that Plug is supposed to be backward compatible to Elixir 1.0, so... :erlang.monotonic_time/0 isn't available.

[whatyouhide]

@mje113 you could do something like we did in Phoenix in this PR (the current Phoenix master code doesn't do this anymore).

[mje113]

@whatyouhide Ah yes, good point. I also came across elixir-lang/elixir#4024 (by you), so it looks like System.monotonic_time/0 is also supported in later releases. Stylistically what's favored in Elixir?

[whatyouhide]

As you said, we can't straight use :erlang.monotonic_time/0 because it's only in newer versions of Erlang; for the same reason, we can't use System.monotonic_time/0 as it's only in newer versions of Elixir 😺 You could check for System.monotonic_time/0 if you follow the pattern in the PR I linked, but I'd say :erlang.monotonic_time/0 is better as the Elixir version requires new versions of both Elixir/Erlang.

[mje113]

Got it, that makes sense. Commit incoming.

[lexmag]

@mje113 please squash everything into single commit.

[lexmag]

We should use :erlang.monotonic_time(:micro_seconds) here and remove convert_time_unit call.

[josevalim]

Let's not do that because we lose precision. The ideal is to always convert later.

[josevalim]

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