fix: KubeletTooManyPods duplicate series error

[skl]

Resolves duplicate series error with KubeletTooManyPods alert:

• Previous query used a per-node pod count, which is expensive and prone to duplicate-series errors with pods of the same name and different uid
• I wanted to avoid depending on uid as many people drop this label due to high cardinality
• kubelet_running_pods metric already exists and gives a per-node count of running pods, alleviating the need to counts the pods per node in the first place

Added a test for multi-node scenario.

Fixes #1015

[jonasbadstuebner]

#1015 (comment)

I'm sorry I didn't answer over the weekend. The label_replace in your proposed solution is incorrect. It's replacing the node name with the IP of the node. kubelet_running_pods already has a node label that has the node name as value, so you can drop the label_replace.