geyser: add metric subscriptions_total

[fanatid]

# HELP subscritpions_total Total number of subscriptions to gRPC service
# TYPE subscritpions_total gauge
subscritpions_total{subscription="accounts"} 0
subscritpions_total{subscription="all"} 2
subscritpions_total{subscription="blocks"} 0
subscritpions_total{subscription="blocks_meta"} 1
subscritpions_total{subscription="entry"} 0
subscritpions_total{subscription="grpc_total"} 1
subscritpions_total{subscription="slots"} 1
subscritpions_total{subscription="transactions"} 0
subscritpions_total{subscription="transactions_status"} 0

[dancamarg0]

great work @fanatid

Just one thing to point out: subscritpions_total has a typo. It's subscriptions and not subscritpions