Although I agree in principle one other aspect that becomes critical in high volume systems is efficiency.

Logging is generally a text stream and in production should only be warning+ levels (imo) so the impact on performance is negligible and really only when something is going wrong.

I want metrics all of the time, potentially 10 or 20 metrics per request/action, a high performance, low network method for sending those metrics with low latency is critical.

We need both systems and both should be treated as tier 1 systems within an organisation. I don't think pushing metrics into a log stream is a scalable architecture.