Expose grace period metrics

[theS1LV3R]

At the moment healthchecks.io expose the following metrics:

Metric	Type	Description
hc_check_up	gauge	Whether the check is currently up (1 for yes, 0 for no).
hc_check_started	gauge	Whether the check is currently started (1 for yes, 0 for no).
hc_tag_up	gauge	Whether all checks with this tag are up (1 for yes, 0 for no).
hc_checks_total	gauge	The total number of checks.
hc_checks_down_total	gauge	The number of checks currently down.

However, in the web-gui it also shows whenever a check is within its grace period (Shows an orange icon).

It would be awesome if that could be exposed as a metric on a per-check basis, something like hc_check_grace