Export Envoy version as string

Title: Export Envoy version as string

Description: The /stats handler exports server.version (and /stats/prometheus likewise envoy_server_version), but it's a bare number (e.g. 9417675), with no structure and is not human readable. In contrast, /server_info has a nicely structured user_agent_build_version block, but that isn't easily able to be scraped with Prometheus. The plain number is sufficient for spotting outliers (e.g. if not all Envoy instances in a fleet have been updated), but isn't pleasant to read.

It'd be nice if Envoy exported a stat that is a human-readable string (e.g. "1.16.0") on /stats and /stats/prometheus. This would make it easier to identify the state of outliers in a fleet, and also easier to analyse relative behaviour or performance when rolling out a new Envoy release.