Document the difference between host and system metrics (#324)

Signed-off-by: ChrsMark <chrismarkou92@gmail.com>
Co-authored-by: Joao Grassi <joao@joaograssi.com>
Co-authored-by: Joao Grassi <joao.grassi@dynatrace.com>
Co-authored-by: Josh Suereth <joshuasuereth@google.com>

docs/resource/host.md

@@ -6,6 +6,9 @@
 
 **Description:** A host is defined as a computing instance. For example, physical servers, virtual machines, switches or disk array.
 
+The `host.*` namespace SHOULD be exclusively used to capture resource attributes.
+To report host metrics, the `system.*` namespace SHOULD be used.
+
 <!-- semconv host -->
 | Attribute  | Type | Description  | Examples  | Requirement Level |
 |---|---|---|---|---|

docs/system/system-metrics.md

@@ -11,6 +11,12 @@ metrics in OpenTelemetry. Consider the [general metric semantic
 conventions](/docs/general/metrics.md#general-metric-semantic-conventions) when creating
 instruments not explicitly defined in the specification.
 
+The `system.*` namespace SHOULD be exclusively used to report hosts' metrics.
+The `system.*` namespace SHOULD only be used when the metrics are collected from within the target system. (physical servers, virtual machines etc).
+Metrics collected from technology-specific, well-defined APIs (e.g. Kubelet's API or container runtimes)
+should be reported under their respective namespace (e.g. k8s.*, container.*).
+Resource attributes related to a host, SHOULD be reported under the `host.*` namespace.
+
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