feat: add metrics to watchdog canister

[maksymar]

This PR adds Prometheus metrics to a watchdog canister.

Metrics response example:

# HELP bitcoin_canister_height Height of the main chain of the Bitcoin canister.
# TYPE bitcoin_canister_height gauge
bitcoin_canister_height 786116 1681915083861
# HELP explorers_number Number of explorers inspected.
# TYPE explorers_number gauge
explorers_number 6 1681915083861
# HELP target_height Target height calculated from the explorers.
# TYPE target_height gauge
target_height 786116 1681915083861
# HELP height_diff Difference between the source and the target heights.
# TYPE height_diff gauge
height_diff 0 1681915083861
# HELP status Status code of the Bitcoin canister height.
# TYPE status gauge
status{height="not_enough_data"} 0 1681915083861
status{height="ok"} 1 1681915083861
status{height="ahead"} 0 1681915083861
status{height="behind"} 0 1681915083861