By enabling the validator monitor, more precise information about the
lifecycle of an attestation is logged at the higher `NOTICE` log level
while current `sent` messages are logged at `INF` instead, since they
are less interesting.
In particular, missed attestations and those that vote for the wrong
head are now detected and logged at NOTICE.
In addition to logging, this feature enables rich metrics around
attestation and sync committee performance - by default, validators are
tracked in aggregate but a detailed mode exists as well
This feature has been available since early Nimbus days, but it has now
been tuned and optimised such that it is safe to enable by default, even
for large setups.
* enable automatic validator monitoring by default
* replace `--validator-monitor-totals` flag with
`--validator-monitor-details` - the detailed mode is disabled by default
* lower "sent" log level to `INF` for several messages - in particular
those that are traced by the validator monitor
This is a retake on #3531 which was later reverted in #3578.
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