Why do I get the following warnings in the Prometheus pod log 'many-to-many matching not allowed: matching labels must be unique on one side'?

Issue

• Metrics created by Prometheus rules like node:node_memory_bytes_available:sum are not evaluated.

• Prometheus is logging warnings like:

  level=warn ts=2020-06-11T21:26:05.335571407Z caller=manager.go:343 component="rule manager" group=node.rules msg="Evaluating rule failed" rule="record: node:node_memory_bytes_available:sum\nexpr: sum by(node) ((node_memory_MemAvailable{job=\"node-exporter\"}) * on(namespace,\n  pod) group_left(node) node_namespace_pod:kube_pod_info:)\n" err="many-to-many matching not allowed: matching labels must be unique on one side"