Clarify metric attributes should be namespaced (#786)

Co-authored-by: Armin Ruech <7052238+arminru@users.noreply.github.com>
Co-authored-by: Chris Mark <chrismarkou92@gmail.com>
Co-authored-by: Liudmila Molkova <limolkova@microsoft.com>

.chloggen/clarify-metric-namespace.yaml

@@ -0,0 +1,5 @@
+change_type: 'enhancement'
+component: metrics
+note: Clarify metric attributes should be namespaced.
+issues: [394]
+subtext:

docs/general/metrics.md

@@ -11,6 +11,7 @@ aliases: [docs/specs/semconv/general/metrics-general]
 
 - [General Guidelines](#general-guidelines)
   - [Name Reuse Prohibition](#name-reuse-prohibition)
+  - [Metric attributes](#metric-attributes)
   - [Units](#units)
   - [Naming rules for Counters and UpDownCounters](#naming-rules-for-counters-and-updowncounters)
     - [Pluralization](#pluralization)
@@ -89,6 +90,26 @@ When introducing a new metric name check all existing schema files to make sure
 the name does not appear as a key of any "rename_metrics" section (keys denote
 old metric names in rename operations).
 
+### Metric attributes
+
+Metric attributes SHOULD follow the general [attribute naming rules](attribute-naming.md).
+In particular, metric attributes SHOULD have a namespace.
+
+Metric attributes SHOULD be added under the metric namespace when their usage and
+semantics are exclusive to the metric.
+
+Examples:
+
+Attributes `mode` and `mountpoint` for metric `system.filesystem.usage`
+should be namespaced as `system.filesystem.mode` and `system.filesystem.mountpoint`.
+
+Metrics can also have attributes outside of their namespace.
+
+Examples:
+
+Metric `http.server.request.duration` uses attributes from the registry such as
+`server.port`, `error.type`.
+
 ### Units
 
 Conventional metrics or metrics that have their units included in