This PR is a follow-up to #6277 and #6290 to add a new prober type to
boulder-observer for monitoring CRLs, making use of the new prober-specific
metrics capability to define the following new metrics:
- `obs_crl_this_update` the Unix timestamp of the CRL's thisUpdate value
- `obs_crl_next_update` the Unix timestamp of the CRL's nextUpdate value
- `obs_crl_revoked_cert_count` the number of certificates listed in the CRL
**Configuration:** Each defined CRL monitor takes a single configuration option,
a URL that specifies the location of the CRL to monitor.
**Metrics:** The three CRL-specific metrics described above are only published
at /metrics if at least one valid monitor is defined in the config.yml. The
metrics have a single label `url` that is set to the URL configured for the
monitor
Phil Porada