Send statistic-get-all only for running daemons

Currently, the Stork agent detects Kea by searching for the Kea Control Agent (CA)
process. The CA configuration file could be expected to contain information about
which kea services are running.

Does stork need to send "statistic-get-all" for both "dhcp4" and "dhcp6" when
only dhcp4 is configured in the CA config? Asking only for statistics from the
active service would reduce the number of errors messages generated by the CA

From backend/agent/promkeaexporter.go

...
        // Request to kea dhcp daemons for getting all stats. Both v4 and v6 is queried because
        // here we do not have knowledge which are active.
        requestData := `{
             "command":"statistic-get-all",
             "service":["dhcp4", "dhcp6"],
             "arguments": {}
        }`
 ...

RT #21508