One strategy do to do that is to trace everything by default and select what to sample later, e.g. https://grafana.com/docs/grafana-cloud/observe-and-act/adapt...
What I’m suggesting is that your apps by default only send metrics to your monitoring system, but that the monitoring system can specifically ask to “upgrade” metrics to traces. Or to log entries.
The same thing with metric cardinality: by default, only report metrics in a fully aggregated manner. But do tell the monitoring system how they can potentially be broken up if needed (i.e., which labels to add).
How does the monitoring system have any of the context to add labels? That would only exist in application memory.
Grafana went the other way - your app exports all labels, and then you selectively aggregate on ingest: https://grafana.com/docs/grafana-cloud/observe-and-act/adapt...
> That may be prohibitively expensive in terms of CPU/network load.
In practice I've not experienced this even on quite high request rates. While it isn't free, exporting everything has been cheap enough that the real cost in dollars spent is basically marginal (it's _storing_ the data that's expensive)
Indeed. If you have a protocol that doesn’t allow exposing that kind of information, then that only lives in application memory. But my suggestion is that it’s exposed.
For your feature to work you need bi-directional communication between the otel receiver and your application - that's still doable in general, but now you want a synchronous "upgrade" to traces.
Now we're talking about a massive performance impact - and you need to somehow cache all otel data locally so they're available for the upgrade and only then submit then.
It is a architecture that's not very smart, honestly. And precisely the reason why you'd simply submit everything and let the receiver figure out which samples it wants to keep - as thorian pointed out earlier.
Yes, because otherwise what you propose requires modifying the binary in-place and that's too big of a security hole for lots of (production) environments. Some variants of that could work with an out-of-process method like Dtrace or eBPF, but that means mutating the kernel, even more of a no-no.