docs: var in singlestat panel. Fixes #8563

docs/sources/features/panels/singlestat.md

@@ -34,7 +34,7 @@ The singlestat panel has a normal query editor to allow you define your exact me
    * `delta` - The total incremental increase (of a counter) in the series. An attempt is made to account for counter resets, but this will only be accurate for single instance metrics. Used to show total counter increase in time series.
    * `diff` - The difference betwen 'current' (last value) and 'first'.
    * `range` - The difference between 'min' and 'max'. Useful the show the range of change for a gauge.
-4. `Postfixes`: The Postfix fields let you define a custom label and font-size (as a %) to appear *after* the value
+4. `Prefix/Postfix`: The Prefix/Postfix fields let you define a custom label and font-size (as a %) to appear *before/after* the value. The `$__name` variable can be used here to use the series name or alias from the metric query.
 5. `Units`: Units are appended to the the Singlestat  within the panel, and will respect the color and threshold settings for the value.
 6. `Decimals`: The Decimal field allows you to override the automatic decimal precision, and set it explicitly.
 

docs/sources/reference/templating.md

@@ -177,6 +177,10 @@ This is used in the WHERE clause for the InfluxDB data source. Grafana adds it a
 
 The `$__timeFilter` is used in the MySQL data source.
 
+### The $__name Variable
+
+This variable is only available in the Singlestat panel and can be used in the prefix or suffix fields on the Options tab. The variable will be replaced with the series name or alias.
+
 ## Repeating Panels
 
 Template variables can be very useful to dynamically change your queries across a whole dashboard. If you want