feat: add prom metrics for stacks block count at previous burn block #2676

Job	Run time
lint	1m 8s
test (api)	10m 25s
test (bns)	3m 21s
test (bns-e2e)	7m 9s
test-rosetta (rosetta)	3m 12s
test (btc-faucet)	1m 35s
test-2_5 (block-zero-handling)	1m 33s
test-rosetta (rosetta-construction)	2m 31s
test (event-replay)	2m 2s
test-rosetta (rosetta-cli-data)	2m 22s
test-2_5 (faucet-stx)	1m 25s
test (rpc)	1m 36s
test-rosetta (rosetta-cli-construction)	2m 24s
test-2_5 (pox-4-btc-address-formats)	2m 21s
test-2_5 (pox-4-delegate-aggregation)	2m 24s
test-2_5 (pox-4-delegate-stacking)	1m 44s
test-2_5 (pox-4-delegate-revoked-stacking)	2m 18s
test-2_5 (pox-4-stack-extend-increase)	2m 18s
test-2_5 (pox-4-rosetta-btc-addr-types)	5m 0s
test-2_5 (pox-4-rosetta-cycle-phases)	1m 24s
test-2_5 (pox-4-rosetta-segwit)	1m 48s
test-2_5 (pox-4-burnchain-stack-stx)	1m 31s
test-2_5 (pox-4-burnchain-delegate-stx)	2m 4s
build-publish	2m 1s
	1h 5m 36s

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