Add clock slot and genesis time metrics

[dapplion]

Motivation

Allow to monitor from Grafana charts the sync status and compute clock drift

Description

After merging the slot chart can be modified to add

(time() - lodestar_genesis_time) / 12

lodestar_clock_slot

To monitor everything in one chart

Also consider adding a new chart with

lodestar_clock_slot - (time() - lodestar_genesis_time) / 12

to measure clock drift

Closes #2462

[codeclimate]

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