monitoring: add assetCollector for asset related metrics

[calvinrzachman]

Enable custom metric for # of minted assets. In the future we will expand this to include additional metrics of interest.

There seems to be some change to the universe stats RPC, upon which the reporting of # of minted assets presently relies. The RPC will not reflect recent changes immediately, but does seem to eventually become consistent.

/ # tapcli --network=regtest --tapddir=/data universe stats
{
    "num_total_assets": "0",
    "num_total_groups": "0",
    "num_total_syncs": "0",
    "num_total_proofs": "0"
}
... some time later
/ # tapcli --network=regtest --tapddir=/data universe stats
{
    "num_total_assets": "8",
    "num_total_groups": "8",
    "num_total_syncs": "0",
    "num_total_proofs": "466"
}

NOTE: This is more or less a mirror of how subasta sets things up.

TODO:

• Check into whether there have been any changes to the functionality of the universe stats RPC and whether a new approach will need to be taken here.

Partially Addresses: https://github.com/lightninglabs/lightning-infra/issues/990

[GeorgeTsagk]

Closing this in favor of #716