Support Action<ReadOnlySpan<byte>> for statistics callbacks

[fabianoliver]

Idea

currently, ConsumerBuilder only allows to registers statistics callbacks as Action<string> .

Would it be possible to allow alternatively registering an Action<ReadOnlySpan<byte>> , which would receive a span of the raw UTF8 string bytes from librdkafka's native memory? Is that something the team would be interested in implementing, and/or willing to accept a PR for it?

Motivation

Quite often, consumers likely aren't interested in the raw string, but would deserialize its JSON contents. System.Text.Json these days works very well based on deserializing raw spans of UTF8 bytes without the need to turn things into a string beforehand.

Avoiding the string allocation would be particularly convenient as the statistics string can be quite large - it tends to be around ~64kb in UTF8 bytes from the samples I've looked at, so roughly 128kb for the actual (UTF-16) c# string. Meaning its quite large, and arguably even worse, large enough to consistently land on the LOH.

This could make a fairly measurable difference - for example, one of the apps I'm running is reliant on running quite a few kafka consumers (I'm aware of the general recommendation to run only 1 consumer per app; however, I've found significant throughput benefits of running several, without having been able to figure out a config for only 1 consumer that would get close). Each of those consumers currently subscribes to statistics every 1000ms. The app ended up allocating almost 8 mb/sec (in the LOH no less) just for statistics callbacks, in spite of only needing to parse/deserialize a tiny subset of the statistics JSON string for the parts it actually cares about. If we had a way of receiving the raw UTF8 bytes, these allocations could almost entirely be avoided.