Add monitoring for public-ip-pool usage

[robert-heinzmann-logmein]

Currently the plugin does only monitor the IP usage in the subnets, not the public ip pools possibly configured in "Bring your own IP" scenarios.

It would be great to also monitor for public ip pool usage

Equivalent ec2-command (see https://docs.aws.amazon.com/cli/latest/reference/ec2/describe-public-ipv4-pools.html):

aws ec2 describe-public-ipv4-pools

Example output

{
    "PublicIpv4Pools": [
        {
            "PoolId": "ipv4pool-ec2-1234567890abcdef0",
            "PoolAddressRanges": [
                {
                    "FirstAddress": "203.0.113.0",
                    "LastAddress": "203.0.113.255",
                    "AddressCount": 256,
                    "AvailableAddressCount": 256
                }
            ],
            "TotalAddressCount": 256,
            "TotalAvailableAddressCount": 256
        }
    ]
}

Interesting metrics

• TotalAddressCount
• TotalAvailableAddressCount