Add grafana dashboard for bench-exchange

[mvines]

Problem

Once bench-exchange is running against a testnet, we have no visual way to view the performance of the exchange.

Proposed Solution

Add pretty graphs and counters for bids, asks, ....
Iterate the dashboard using feedback from @aeyakovenko until it's looking solid for his needs.

[mvines]

@jackcmay - you're probably the best person to take a first swag at defining exchange metrics to add to Grafana.

[sakridge]

I added this one:

solana/bench-exchange/src/bench.rs

Line 334 in 809b051

solana_metrics::submit(

But not sure how to modify the dashboard to include it.

[aeyakovenko]

@sakridge we need some for client side requests submitted and in the contract, requests processed. One for bid/ask price updates and matched.

[mvines]

All done now