Transform Example: Change context to datapoint (open-telemetry#27041

)

**Description:** 
- Fixing documentation for incorrect transform context.
- Correcting `resource.attributes["k8s_namespace_name"` to be correct
`.` separated name.
- Removing `resource.attributes["k8s.cluster.name"]` as it's not an
available attribute.  

**Link to tracking Issue:**

open-telemetry#27012 (comment)

**Testing:** 
Works in my own otel-collector.

**Documentation:** 
All documentation updates here.

exporter/prometheusexporter/README.md

@@ -85,12 +85,11 @@ This is not a common pattern, and we recommend copying the most common resource
 processor:
  transform:
  metric_statements:
- - context: metric
+ - context: datapoint
  statements:
- - set(attributes["namespace"], resource.attributes["k8s_namespace_name"])
+ - set(attributes["namespace"], resource.attributes["k8s.namespace.name"])
  - set(attributes["container"], resource.attributes["k8s.container.name"])
  - set(attributes["pod"], resource.attributes["k8s.pod.name"])
- - set(attributes["cluster"], resource.attributes["k8s.cluster.name"])
 ```
 
 After this, grouping or selecting becomes as simple as:

exporter/prometheusremotewriteexporter/README.md

@@ -122,12 +122,11 @@ This is not a common pattern, and we recommend copying the most common resource
 processor:
  transform:
  metric_statements:
- - context: metric
+ - context: datapoint
  statements:
- - set(attributes["namespace"], resource.attributes["k8s_namespace_name"])
+ - set(attributes["namespace"], resource.attributes["k8s.namespace.name"])
  - set(attributes["container"], resource.attributes["k8s.container.name"])
  - set(attributes["pod"], resource.attributes["k8s.pod.name"])
- - set(attributes["cluster"], resource.attributes["k8s.cluster.name"])
 ```
 
 After this, grouping or selecting becomes as simple as: