Add beacon node count metrics

[jhett12321]

The validator client provides a useful summary of beacon node health in the slot start log entries.

This PR exposes these node count values as a new validator metric, so they can be used as potential health checks.

# HELP validator_client_node_counts Number of connected beacon nodes and their status
# TYPE validator_client_node_counts gauge
validator_client_node_counts{status="bad"} 0.0
validator_client_node_counts_created{status="bad"} 1718820863.0
validator_client_node_counts{status="good"} 2.0
validator_client_node_counts_created{status="good"} 1718820863.0
validator_client_node_counts{status="viable"} 0.0
validator_client_node_counts_created{status="viable"} 1718820863.0

[github-actions]

Unit Test Results

         9 files  ±0    1 328 suites  ±0   32m 52s ⏱️ -51s
  4 998 tests ±0    4 650 ✔️ ±0  348 💤 ±0  0 ❌ ±0 
20 850 runs  ±0  20 446 ✔️ ±0  404 💤 ±0  0 ❌ ±0 

Results for commit 242d6fe. ± Comparison against base commit f30f670.