Update to 0.21.2

Changelog:

• added check-single-keys and make check-keys not use KEYS
• add total_system_memory metric
• environment variables for all command line options
• more persistence related metrics

PR: 232321
Submitted by: Volodymyr Kostyrko <arcade@b1t.name> (maintainer)
Sponsored by: Netzkommune GmbH