Common variable queries should include a method for showing all values

Here's a message from community slack: https://influxcommunity.slack.com/archives/CH8RV8PK5/p1631632955155400

The pattern is essentially:

Add a custom row to your variable with a value of your choosing to represent all values (`*`)
```
import "influxdata/influxdb/schema"
import "array"

variable_query = schema.measurements(bucket: "telegraf")
all_values = array.from(rows: [{_value: "*"}])

union(tables: [variable_query, all_values])
```

then in my query, i do something in the filter like:
```
|> filter(fn: (r) => if v.my_variable == "*" then true else r.tag == v.my_variable)
```

##### Relevant URLs
- https://docs.influxdata.com/influxdb/cloud/visualize-data/variables/common-variables/