mempool: Add metric to measure how many times a tx was received

[hvanz]

Feature Request

Related to #11. Prerequisite for #613.

Problem Definition

Currently transactions are propagated to all peers except to the peer that sent the transaction. Then a transaction may reach a node multiple times. A metric for counting the number a times a transaction was received by a node would help to improve the gossiping mechanism.

Proposal

Exposing one metric for each tx would add a huge number of Prometheus entries at each measurement. A better way would be to add a new metric of type histogram with sizes of buckets increasing exponentially. For example, buckets of size 1 (that is, number of transactions received at most once), 2, 4, 8, and more than 8.