add hints related graphs

[vladzcloudius]

HEAD: dd390a6

Feature requests
Add hints related graphs.
Hints metrics are coordinator metrics however it also makes sense to have them not too far from the replica side metrics because they are correlated with these.

[tzach]

@vladzcloudius please include the relevant metrics names.

[vladzcloudius]

@tzach scylla_hints_for_views_manager_* and scylla_hints_manager_*

[vladzcloudius]

@amnonh ping.

I looked at the current dashboards state in the master: it seems to make sense to start with adding the scylla_hints_manager_writes and scylla_hints_manager_sent graphs in the Coordinator group on the Detailed dashboard and adding scylla_hints_for_views_manager_writes and scylla_hints_for_views_manager_sent graphs in the View section.