Add attestation duty slot metric (#2704)

## Issue Addressed

Resolves #2521 

## Proposed Changes

Add a metric that indicates the next attestation duty slot for all managed validators in the validator client.

validator_client/src/lib.rs

@@ -422,6 +422,7 @@ impl<T: EthSpec> ProductionValidatorClient<T> {
             },
             spec: context.eth2_config.spec.clone(),
             context: duties_context,
+            enable_high_validator_count_metrics: config.enable_high_validator_count_metrics,
         });
 
         // Update the metrics server.