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/http_metrics/metrics.rs

@@ -172,6 +172,12 @@ lazy_static::lazy_static! {
         "Duration to obtain a signature",
         &["type"]
     );
+
+    pub static ref ATTESTATION_DUTY: Result<IntGaugeVec> = try_create_int_gauge_vec(
+        "vc_attestation_duty_slot",
+        "Attestation duty slot for all managed validators",
+        &["validator"]
+    );
 }
 
 pub fn gather_prometheus_metrics<T: EthSpec>(