vmui: provide an UI for labels' exploration

[valyala]

Is your feature request related to a problem? Please describe.

VictoriaMetrics provides the following API endpoints for labels' exploration:

• /api/v1/labels - this endpoint returns label names across time series. It supports filtering series by time range and series selector. See these docs.
• /api/v1/label/.../values - this endpoint returns values for a particular label across time series. It supports filtering series by time range and series selector. See these docs.

Describe the solution you'd like

It would be great if vmui could provide an UI for labels' exploration. The UI should provide the following functionality:

• Ability to select a particular label from the auto-completion list of available label names and see values for this label.
• Ability to limit the number of exposed label values, since some labels can have millions of unique values. By default the limit should be set to 100 in order to prevent from UI overload.
• Ability to specify a particular time range.
• Ability to specify time series selector for filtering unneeded time series.

The UI for labels' exploration can be linked to cardinality explorer. For example, every exposed label value can provide a link to cardinality explorer with the {label="value"} filter. On the other side, every row at the top label names with the highest number of series table at cardinality explorer can provide a link to labels' exploration UI for the given label name.

[valyala]

The related feature request, which has been already implemented - #3386 . It lowers the priority of this feature request.