An event log is typically associated with a long-lived object, like an
RPC connection.  Printf calls record events; Errorf calls record
events marked as errors.  The HTTP endpoint /debug/events organizes
event logs by family (usually the Go type name) and by
time-since-last-error.  The expanded view shows recent events in the
log and the call stack where the event log was created.

Change-Id: I3461e0d63f39ce6495e16300299048e572b3aa19
Reviewed-on: https://go-review.googlesource.com/12025Reviewed-by: David Symonds <dsymonds@golang.org>