Kea-exporter

- Fixes missing version bump in pyproject.toml

- Converto PEP517 build system and use poetry-core for the build
- Gracefully handle unknown metrics and nudge users to report issues
for them

- Ignore cumulative-assigned-{nas,pds,addresses} introduced some time
in the 1.7.x series
- Export the subnet id as an additional label

- Fixes a typo in metric exposing the pkt6-parse-failed counter
- Clarifies the minimum python version as 3.6

- Kea-exporter now consumes control sockets instead of config paths
- The configuration is being queried from the control socket, allowing
for more complex configurations, like those using includes.
- Use makefile() to read from socket, thereby supporting larger setups

- Fix script entrypoint