Add Etna P-chain metrics

[StephenButtolph]

Why this should be merged

Adds useful metrics:

# HELP avalanche_platformvm_gas_consumed Cumulative amount of gas consumed by transactions
# TYPE avalanche_platformvm_gas_consumed counter
avalanche_platformvm_gas_consumed{chain="P"} 482042
# HELP avalanche_platformvm_gas_capacity Minimum amount of gas that can be consumed in the next block
# TYPE avalanche_platformvm_gas_capacity gauge
avalanche_platformvm_gas_capacity{chain="P"} 999333
# HELP avalanche_platformvm_active_sovs Number of active Subnet only Validators
# TYPE avalanche_platformvm_active_sovs gauge
avalanche_platformvm_active_sovs{chain="P"} 9
# HELP avalanche_platformvm_excess Excess usage of a resource over the target usage
# TYPE avalanche_platformvm_excess gauge
avalanche_platformvm_excess{chain="P",resource="gas"} 667
avalanche_platformvm_excess{chain="P",resource="validators"} 0
# HELP avalanche_platformvm_price Price (in nAVAX) of a resource
# TYPE avalanche_platformvm_price gauge
avalanche_platformvm_price{chain="P",resource="gas"} 1
avalanche_platformvm_price{chain="P",resource="validators"} 1

How this works

Adds a metrics.Block that is passed into metrics.Metrics#MarkAccepted which includes useful information related to the Etna state.

How this was tested

• Ran node on the devnet and metrics are exposed as expected.