k8sprocessor: fix setting attributes on metrics passed from agent

[dmitryax]

Bug description:
When otel-collector running as an agent with k8s processor in passthrough mode sets "k8s.pod.id" resource label to fix metrics source for further enrichment, standalone collector miss that attribute and doesn't enrich metrics as expected.

Testing: Added a test to cover the bug

[codecov]

Codecov Report

Merging #836 into master will increase coverage by 16.18%.
The diff coverage is 100.00%.

@@             Coverage Diff             @@
##           master     #836       +/-   ##
===========================================
+ Coverage   72.00%   88.18%   +16.18%     
===========================================
  Files          14      231      +217     
  Lines         600    12406    +11806     
===========================================
+ Hits          432    10940    +10508     
- Misses        144     1111      +967     
- Partials       24      355      +331     

Flag	Coverage Δ
#integration	72.00% <ø> (ø)
#unit	88.00% <100.00%> (?)

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
processor/k8sprocessor/processor.go	96.22% <100.00%> (ø)
...sformprocessor/operation_aggregate_label_values.go	100.00% <0.00%> (ø)
...resourcedetectionprocessor/internal/aws/ec2/ec2.go	90.90% <0.00%> (ø)
exporter/awsxrayexporter/translator/aws.go	96.74% <0.00%> (ø)
receiver/awsxrayreceiver/receiver.go	100.00% <0.00%> (ø)
exporter/carbonexporter/metricdata_to_plaintext.go	86.11% <0.00%> (ø)
exporter/sentryexporter/transport.go	31.57% <0.00%> (ø)
...oudlogserviceexporter/metricsdata_to_logservice.go	100.00% <0.00%> (ø)
receiver/receivercreator/runner.go	53.33% <0.00%> (ø)
exporter/splunkhecexporter/tracedata_to_splunk.go	100.00% <0.00%> (ø)
... and 213 more

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