Document counter type

docs/user/metrics/counter.md

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+# Counter
+
+Count how often something happens.
+
+## Methods
+
+* `add(amount)` - Increase the counter by `amount`.
+
+## Limits
+
+* Only increments, saturates at the limits of a 32-bit unsigned integer
+
+## Examples
+
+* How often a certain button was pressed?
+
+## Recorded errors
+
+* None.