Add a metric counting the number of negotiated packfiles

We have recently added a set of metrics that count which Git features are used during a git-clone(1) client operation for both the smarthttp and ssh services. While these metrics can already be put into a relation to the overall number of clones for smarthttp as the PostUploadPack RPC is used for packfile generation only, the ssh service does both reference discovery and packfile negotiation in the SSHUploadPack RPC. As a result, the number of actual packfile negotiations get drowned out by reference discoveries, so we can't really tell how many clients use any of the tracked metrics.

Add a new metric to explicitly track how many packfile negotiations were performed to be able to tell both kinds of requests to SSHUploadPack apart.