Cleanup bench-exchange (#3919)

* bench-exchange changes

Generate new keypair for each bench-exchange
Add metrics
Tweak network sleep parameters for better reliability.

* Bench-exchange contract metrics

programs/exchange_api/src/lib.rs

@@ -2,6 +2,9 @@ pub mod exchange_instruction;
 pub mod exchange_processor;
 pub mod exchange_state;
 
+#[macro_use]
+extern crate solana_metrics;
+
 use solana_sdk::pubkey::Pubkey;
 
 pub const EXCHANGE_PROGRAM_ID: [u8; 32] = [