core: add API to measure event loop idle time

The API addition `uv_metrics_idle_time()` is a thread safe call that
allows the user to retrieve the amount of time the event loop has spent

core: add API to measure event loop idle time

The API addition `uv_metrics_idle_time()` is a thread safe call that
allows the user to retrieve the amount of time the event loop has spent
in the kernel's event provider (i.e. poll). It was done this way to
allow retrieving this value without needing to interrupt the execution
of the event loop. This option can be enabled by passing
`UV_METRICS_IDLE_TIME` to `uv_loop_configure()`.

One important aspect of this change is, when enabled, to always first
call the event provider with a `timeout == 0`. This allows libuv to know
whether any events were waiting in the event queue when the event
provider was called. The importance of this is because libuv is tracking
the amount of "idle time", not "poll time". Thus the provider entry time
is not recorded when `timeout == 0` (the event provider never idles in
this case).

While this does add a small amount of overhead, when enabled, but the
overhead decreases when the event loop has a heavier load. This is
because poll events will be waiting when the event provider is called.
Thus never actually recording the provider entry time.

Checking if `uv_loop_t` is configured with `UV_METRICS_IDLE_TIME` always
happens in `uv__metrics_set_provider_entry_time()` and
`uv__metrics_update_idle_time()`. Making the conditional logic wrapping
each call simpler and allows for instrumentation to always hook into
those two function calls.

Rather than placing the fields directly on `uv__loop_internal_fields_t`
add the struct `uv__loop_metrics_t` as a location for future metrics API
additions.

Tests and additional documentation has been included.

PR-URL: https://github.com/libuv/libuv/pull/2725
Reviewed-By: Fedor Indutny <fedor.indutny@gmail.com>
Reviewed-By: Santiago Gimeno <santiago.gimeno@gmail.com>
Reviewed-By: Jameson Nash <vtjnash@gmail.com>

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doc: move TOC to a dedicated document

PR-URL: https://github.com/libuv/libuv/pull/1246
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Santiago Gimeno <santiago.gimeno@gmai

doc: move TOC to a dedicated document

PR-URL: https://github.com/libuv/libuv/pull/1246
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Santiago Gimeno <santiago.gimeno@gmail.com>

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