dhcp_server: T6852: Add op mode command "show dhcpv6 server statistics"

[natali-rs1985]

Change Summary

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Code style update (formatting, renaming)
• Refactoring (no functional changes)
• Migration from an old Vyatta component to vyos-1x, please link to related PR inside obsoleted component
• Other (please describe):

Related Task(s)

• https://vyos.dev/T6852

Related PR(s)

Component(s) name

op mode

Proposed changes

added command show dhcpv6 server statistics

How to test

set interfaces ethernet eth1 address '2001:db8::1/64'

set service dhcpv6-server shared-network-name NET1 interface 'eth1'
set service dhcpv6-server shared-network-name NET1 subnet 2001:db8::/64 option name-server '2001:db8::ffff'
set service dhcpv6-server shared-network-name NET1 subnet 2001:db8::/64 range 1 start '2001:db8::100'
set service dhcpv6-server shared-network-name NET1 subnet 2001:db8::/64 range 1 stop '2001:db8::199'
set service dhcpv6-server shared-network-name NET1 subnet 2001:db8::/64 subnet-id '1'
commit

vyos@vyos:~$ show dhcpv6 server statistics
Pool    Size    Leases    Available    Usage
------  ------  --------  -----------  -------
NET1    154     1         153          1%

vyos@vyos:~$ show dhcpv6 server statistics pool
Possible completions:
  NET1                  Show DHCPv6 server statistics for a specific pool


vyos@vyos:~$ show dhcpv6 server statistics pool NET1
Pool    Size    Leases    Available    Usage
------  ------  --------  -----------  -------
NET1    154     1         153          1%

Smoketest result

Checklist:

• I have read the CONTRIBUTING document
• I have linked this PR to one or more Phabricator Task(s)
• I have run the components SMOKETESTS if applicable
• My commit headlines contain a valid Task id
• My change requires a change to the documentation
• I have updated the documentation accordingly

[github-actions]

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[github-actions]

CI integration 👍 passed!

Details

CI logs

• CLI Smoketests (no interfaces) 👍 passed
• CLI Smoketests (interfaces only) 👍 passed
• Config tests 👍 passed
• RAID1 tests 👍 passed
• TPM tests 👍 passed