Collect clock_delta metric

[rosik]

Membership has a feature for measuring clock drift. The Cartridge uses it for showing an issue if the diff exceeds a threshold.

But even if the issue isn't shown, the exact value of clock_delta may still be interesting for matching logs of two different instances.

Let's expose this metric by default.

Check out this code.

https://github.com/tarantool/cartridge/blob/cc607f5a6508449608f3953a3f93669e8c8c4ab0/cartridge/issues.lua#L375-L398

But, perhaps, in metrics, min and max values should be separate.

See also:

• Measure clock drift membership#6
• Add information about clock difference between members  membership#4