[Feature Request] legend: Provide key and value as arguments for format

[adschmu]

Description

With #3662 fixed, one could provide both the identifier (key in data.names) and the friendly name (value in data.names) as arguments in the legend.format function. If data.names is not used in the config, both arguments could just contain the same value.

  data: {
    names: {"data": "Detailed Name"},
    columns: [
      ["data", 91.4]
    ],
    type: "gauge"
  },
  legend: {
      format: function (key, value) {
        if (key == "data")
          return value.substr(0 ,5) + "..."; // Would return "Detai..."
        else
          return value;
      },
      tooltip: true
  }

This would allow to use systematically named "keys"/"identifiers" and then apply different formattings based on them.

Imagine variable language-localized friendly names ("values") loaded from an external source and invariant identifiers ("keys"). One could then easily switch based on the known keys to shorten/adjust the localized values without having to parse the latter.