Unable to setup custom metrics

Using Istio 1.8.2
Followed the instructions here Istio / Customizing Istio Metrics, with some changes to try to create a new metric rather than altering an existing one.

My configuration is:

    telemetry:
      v2:
        prometheus:
          configOverride:
            inboundSidecar:
              definitions:
                - name: test_metric
                  type: COUNTER
                  value: "1"
              metrics:
                - name: test_metric
                  dimensions:
                    request_host: request.host
            outboundSidecar:
              definitions:
                - name: test_metric
                  type: COUNTER
                  value: "1"
              metrics:
                - name: test_metric
                  dimensions:
                    request_host: request.host
            gateway:
              definitions:
                - name: test_metric
                  type: COUNTER
                  value: "1"
              metrics:
                - name: test_metric
                  dimensions:
                    request_host: request.host

My envoy config now has a large number of istio.stats filter entries, with the following information:

              "configuration": {
               "@type": "type.googleapis.com/google.protobuf.StringValue",
               "value": "{\"definitions\":[{\"name\":\"test_metric\",\"type\":\"COUNTER\",\"value\":\"1\"}],\"metrics\":[{\"dimensions\":{\"request_host\":\"request.host\"},\"name\":\"test_metric\"}]}\n"
              }

Which I’m assuming isn’t correct, as there are no test_metric or istio_test_metric stats being output by envoy - however I haven’t been able to find a good example of what this should look like, and there am mostly extrapolating from the docs, which only talk about altering an existing metric.

If anyone has any insight into what I’m doing incorrectly here that would be greatly appreciated - thanks!

Further to this, I seem to be seeing the following error

expr_evaluate value error: No value with name "request" found in Activation