We would like to expose a stats API to extensions through Wasm that is compatible with a general purpose OpenCensus/OpenTelemetry interfaces. Generally, these provide an ability to separate tag keys from tag values (e.g. https://opencensus.io/tag/). Envoy currently compresses all tags into a single long string, and expects custom tag extractors for post-processing. This causes two issues:

1. Regex overhead. Separating key=value pairs requires extra static bootstrap config (it is neither xDS compatible, nor modifiable by extensions), and extra parsing overhead since these are not simple prefix matches.
2. Symbol tables cannot distinguish static tag keys from the dynamic/per-request tag values. OpenCensus, for example, maintains a global symbol table for keys but not values, since registering new values at request time would cause lock contention. Envoy, however, would either inline all stats symbols or none of them, there is no way to distinguish between keys and values.

This is a feature request to extend the internal stats API to allow registering stat tag keys so that they can be automatically extracted on export without regex parsing, and symbol table gains awareness of the keys vs values.

cc @jmarantz @mandarjog @jplevyak