Number Widget does not support `Show documents for value`

## Expected Behavior
When opening the value actions for a number widget the `Show documents for values` should
be enabled.

## Current Behavior
When opening value action for the number widget `Show documents for values` is disabled.

## Possible Solution
> my guess is that the field type is not properly provided at this point
`@dennisoelkers`

## Steps to Reproduce (for bugs)
1. Create a message counter widget
2. click on the value
3. See disabled `Show documents for value`

## Context
https://community.graylog.org/t/graylog3-2-dashboard-changes/13877/8

## Your Environment
* Graylog Version: 3.2.0