Clarify when "count" is used instead of pluralization

The existing pluralization rule does not explain why we have
metrics like `system.processes.count`.

This adds a paragraph to explain when `count` is used instead
of pluralization.

Resolves #2553

specification/metrics/semantic_conventions/README.md

@@ -105,6 +105,17 @@ should not be pluralized, even if many data points are recorded.
 * `system.paging.faults`, `system.disk.operations`, and `system.network.packets`
 should be pluralized, even if only a single data point is recorded.
 
+#### Exception
+
+If the value being recorded represents the count of concepts signified
+by the namespace then the metric should be named `count` (within its namespace).
+The pluralization rule does not apply in this case.
+
+For example if we have a namespace `system.processes` which contains all metrics related
+to the processes then to represent the count of the processes we can have a metric named
+`system.processes.count`. The suffix `count` here indicates that it is the count of
+`system.processes`.
+
 ## General Metric Semantic Conventions
 
 The following semantic conventions aim to keep naming consistent. They