[mempool] Prevent double decrements of upstream peers metric
[mempool] Prevent double decrements of upstream peers metric

Closes: #8676

libra/libra,libra/libra,aptos-labs/aptos-core,libra/libra,libra/libra,aptos-labs/aptos-core,aptos-labs/aptos-core,aptos-labs/aptos-core,libra/libra,aptos-labs/aptos-core,aptos-labs/aptos-core,aptos-labs/aptos-core