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/cli.rs

@@ -231,6 +231,15 @@ pub fn cli_app<'a, 'b>() -> App<'a, 'b> {
                     address of this server (e.g., http://localhost:5064).")
                 .takes_value(true),
         )
+        .arg(
+            Arg::with_name("enable-high-validator-count-metrics")
+                .long("enable-high-validator-count-metrics")
+                .help("Enable per validator metrics for > 64 validators. \
+                    Note: This flag is automatically enabled for <= 64 validators. \
+                    Enabling this metric for higher validator counts will lead to higher volume \
+                    of prometheus metrics being collected.")
+                .takes_value(false),
+        )
         /*
          * Explorer metrics
          */