Let me add to this thread my three bits. Speaking of Envoy, yes, when attached to your application it adds a lot of useful features from observability bucket, e.g. network level statistics and tracing.
Here is the question, have you considered running your legacy apps inside service mesh, like Istio ?.
Istio simplifies deployment and configuration of Envoy for you. It injects sidecar container (istio-proxy, in fact Envoy instance) to your Pod application, and gives you these extra features like a set of service metrics out of the box*.
Example: Stats produced by Envoy in Prometheus format, like istio_request_bytes
are visualized in Kiali Metrics dashboard for inbound traffic as request_size
(check screenshot)
*as mentioned by @David Kruk, you still needs to have Prometheus server deployed in your cluster to be able to pull these metrics to Kiali dashboards.
You can learn more about Istio here. There is also a dedicated section on how to visualize metrics collected by Istio (e.g. request size).
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