Use function capture for attaching `handle_event/4`

[nick96]

I hope you don't mind me making a PR without an associated issue. I couldn't find an existing one and this seemed like a relatively easy fix.

The documentation for :telemetry.attach/4 states that function capture should be used for the event handler, rather than local function capture for performance reasons. Using local function capture also leads to warnings like:

https://hexdocs.pm/telemetry/telemetry.html#attach/4
[info] The function passed as a handler with ID {Telemetry.Metrics.ConsoleReporter, [:phoenix, :router_dispatch, :start], #PID<0.576.0>} is a local function.
This means that it is either an anonymous function or a capture of a function without a module specified. That may cause a performance penalty when calling that handler. For more details see the note in `telemetry:attach/4` documentation.

Unfortunately, to allow using function capture, I've had to make handle_event/4 a public function.

[codecov-commenter]

Codecov Report

Merging #107 (b9a1637) into main (084a430) will not change coverage. The diff coverage is 100.00%.

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[josevalim]

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