Add claimable reward token metrics to the Prometheus metrics endpoint

[spireblockchain]

Add the number of reward tokens that may be claimed to the metrics endpoint (and any other reward metrics that may be relevant).

[spireblockchain]

Possible format:
ergo_oracle_rewards_claimable{oracle_address="addr1...."} 30
ergo_oracle_rewards_claimable{oracle_address="addr2...."} 31