[ENH] Add user agent to rust frontend traces (#4323)

## Description of changes

Adds user agent to rust frontend traces. Previously this was included in
the legacy frontend traces, but we had not included it in the rust
frontend traces (yet).

## Test plan

Tested locally with tilt environment, confirmed traces included user
agent in jaeger.

## Documentation Changes
n/a

Author: jasonvigil

Diff for: rust/frontend/src/tower_tracing.rs

@@ -24,6 +24,11 @@ impl<B> MakeSpan<B> for RequestTracing {
             .get(header::HOST)
             .map_or("", |h| h.to_str().unwrap_or(""));
 
+        let user_agent = request
+            .headers()
+            .get(header::USER_AGENT)
+            .map_or("", |h| h.to_str().unwrap_or(""));
+
         let name = format!("{} {}", request.method(), http_route);
 
         tracing::span!(
@@ -35,6 +40,7 @@ impl<B> MakeSpan<B> for RequestTracing {
             http.version = ?request.version(),
             http.host = %host,
             http.status_code = tracing::field::Empty,
+            http.user_agent = %user_agent,
             otel.name = name,
             otel.status_code = tracing::field::Empty,
         )