Add accurate units for metrics

Some keys in configuration.json are gauges, to accurately represent these we should also add the correct unit. Currently when a unit is not specified it will be generated with "unit": "none" (MB-60431).

For example, checkpoint max size is in bytes, currently the unit will read none: 

"kv_ep_checkpoint_max_size": {

        "added": "7.0.0",

        "help": "Max size (in bytes) of a single checkpoint. '0' for EPEngine auto-setup.",

        "stability": "volatile",

        "type": "gauge",

        "unit": "none"

    },