Better metrics & logs around indexing health / ETA

[tkporter]

ETA calculation is broken for some types of cursors - this ticket includes fixing all of these or deciding some alternative

• We have the indexed_height metric which made complete sense when we just had sliding windows of blocks. However with the sequence-aware cursors, this doesn't make as much sense (or does it ?)
  • We could find an alternative metric?
  • Or for sequence-aware cursors just return the max(target snapshot block, current indexing block)
  • The indexed height probably gets messed up by rewinds?
  • Should we instead / also use timestamps as suggested here? Sequence-aware indexing refactor #3262 (comment)
• It doesn't work for the backwards cursor, we should either implement this or justify it not existing (related: Index time to sync is incorrect for the backwards cursor #2504)
• Log health should be really clear, both in logs and metrics. Should we have a counter for indexing rewinds or errors?