General signer metrics

[ximon18]

Do we want to expose as Prometheus metrics things like number of keys, number of signing operations, details of how quickly the HSM reponds e.g. histogram of key generation or signing speeds, counts of certain types of error, percentage of HSM space used (e.g. maybe there are standardized counters for max keys possible?).

[ximon18]

For connection to external HSMs e.g. via PKCS#11 and/or KMIP interfaces we also need a way to know if the connection is still healthy or not. One way to do this would be to track last successful and last failed connection/request timestamps as Prometheus metrics.