Align system.cpu.state's definition with this of process.cpu.state

Signed-off-by: ChrsMark <chrismarkou92@gmail.com>

.chloggen/add_cpu_state_constraint.yaml

@@ -0,0 +1,22 @@
+# Use this changelog template to create an entry for release notes.
+#
+# If your change doesn't affect end users you should instead start
+# your pull request title with [chore] or use the "Skip Changelog" label.
+
+# One of 'breaking', 'deprecation', 'new_component', 'enhancement', 'bug_fix'
+change_type: 'enhancement'
+
+# The name of the area of concern in the attributes-registry, (e.g. http, cloud, db)
+component: 'system'
+
+# A brief description of the change. Surround your text with quotes ("") if it needs to start with a backtick (`).
+note: Align `system.cpu.state`'s definition with this of `process.cpu.state`.
+
+# Mandatory: One or more tracking issues related to the change. You can use the PR number here if no issue exists.
+# The values here must be integers.
+issues: [563]
+
+# (Optional) One or more lines of additional information to render under the primary note.
+# These lines will be padded with 2 spaces and then inserted directly into the document.
+# Use pipe (|) for multiline entries.
+subtext:

docs/system/system-metrics.md

@@ -87,7 +87,7 @@ This metric is [recommended][MetricRecommended].
 | Attribute  | Type | Description  | Examples  | Requirement Level |
 |---|---|---|---|---|
 | `system.cpu.logical_number` | int | The logical CPU number [0..n-1] | `1` | Recommended |
-| `system.cpu.state` | string | The state of the CPU | `idle`; `interrupt` | Recommended |
+| `system.cpu.state` | string | The CPU state for this data point. A system's CPU SHOULD be characterized _either_ by data points with no `state` labels, _or only_ data points with `state` labels. | `idle`; `interrupt` | Recommended |
 
 `system.cpu.state` has the following list of well-known values. If one of them applies, then the respective value MUST be used, otherwise a custom value MAY be used.
 
@@ -116,7 +116,7 @@ This metric is [recommended][MetricRecommended].
 | Attribute  | Type | Description  | Examples  | Requirement Level |
 |---|---|---|---|---|
 | `system.cpu.logical_number` | int | The logical CPU number [0..n-1] | `1` | Recommended |
-| `system.cpu.state` | string | The state of the CPU | `idle`; `interrupt` | Recommended |
+| `system.cpu.state` | string | The CPU state for this data point. A system's CPU SHOULD be characterized _either_ by data points with no `state` labels, _or only_ data points with `state` labels. | `idle`; `interrupt` | Recommended |
 
 `system.cpu.state` has the following list of well-known values. If one of them applies, then the respective value MUST be used, otherwise a custom value MAY be used.
 

model/metrics/system-metrics.yaml

@@ -34,7 +34,7 @@ groups:
               value: 'interrupt'
             - id: steal
               value: 'steal'
-        brief: "The state of the CPU"
+        brief: "The CPU state for this data point. A system's CPU SHOULD be characterized *either* by data points with no `state` labels, *or only* data points with `state` labels."
         examples: ["idle", "interrupt"]
       - id: logical_number
         type: int