Include timing information in leadership tracer events

@karknu suggested that this could help a node operator determine if their node needs more resources. EG If the `TraceNodeNotLeader` was coming out near the end of each slot, that suggests that their machine is just barely managing to check each slot -- it'll likely start missing some checks if eg some additional CPU load arrived.

Start by locating the leadership tracer events, including TraceNodeNotLeader, and read how their event data is produced and consumed. Determine what timing information should be attached so node operators can assess whether checks occur near the end of a slot; the work is done when the relevant events expose that information consistently.

Written by the indexing model from the issue text.