Feature request: Template variables for dashboard UID, panel ID and template Index

[oplehto]

Why is this needed:
Having template variables for the dashboard UID, panel ID and template index would accomplish at least two things:

1. Be able to pass fine-grained information about the source of the query to the datasource, for example by using an SQL comment. This will make it possible to quickly trace back problematic queries to the originating panel or template variable.
2. Make it possible to pass enough context in data links to be able to refer back to the exact panel where the query was coming from

What would you like to be added:
3 template variables:
$__dashboard_uid - The UID of the current dashboard
$__panel_id - The panel ID of the current panel (only for queries in panels)
$__template_idx - The index of the template variable (only for template variable queries)

Who is this feature for?
Advanced users with large dashboards and datasource developers who want to include fine-grained information of Grafana usage into the datasource's query log. Possibly others as well?