add metadata to tooltip

<!-- Please only use this template for submitting feature requests --> **Why is this needed**: When showing data on state timelines, I need to be able to see some clarifying info on hover to e.g. understand the cause of the state or how a potential issue was resolved. i would like to have a string in tooltip for each datapoint, plus the actual data value **What would you like to be added**: I want to be able to specify fields to show in the tooltip on hover. Example from the community panel call Gannt, where this feature is indicated:  It is specified based on column name like this:  **Who is this feature for?** This is relevant for all state timelines where quickly getting more info about a specific state is necesdsary without going to another dashboard for it. Link to the forum topic, where other s show interest in a similar feature (or something that this feature would resolve): https://community.grafana.com/t/grafana-tooltip-customization/66221/10