Add trace/span attributes for internal error status and latency crossing the Envoy layer

Title: Add trace/span attributes for internal error status and latency crossing the Envoy layer

Description:

I'm working with a team at my org who's trying to prove to skeptical internal dev groups that Envoy is faster than older proxy setups. We've had good success showing off performance improvements by tracing other tools. We'd like to have span attributes in Envoy that show the time it took for Envoy to process a request internally, and another attribute to record if there's an error internal to Envoy. That would let us track Envoy's impact on performance and errors in our tracing systems.

Relevant Links:

We've enabled spawn_upstream_span as documented here, but tis still only shows key attributes like duration_ms in relation to the proxied call. We don't see a separate span attribute that shows internal latency within Envoy itself.