Came across this which gives good insight into the 4 golden signals for a top-level health tracking: https://blog.netsil.com/the-4-golden-signals-of-api-health-a...

One thing of note in the graph is the tracking of response size. This would be very useful for 200 responses with "Error" in the text. Because then the response size would drop drastically below a normal successful response payload size.

In addition to Latency, Error Rates, Throughput and Saturation , folks like Brendan Gregg @ Netflix have recommended tracking capacity.