Proxy error reworking (#6453)

## Problem

Taking my ideas from https://github.com/neondatabase/neon/pull/6283 and
doing a bit less radical changes. smaller commits.

We currently don't report error classifications in proxy as the current
error handling made it hard to do so.

## Summary of changes

1. Add a `ReportableError` trait that all errors will implement. This
provides the error classification functionality.
2. Handle Client requests a strongly typed error
    * this error is a `ReportableError` and is logged appropriately
3. The handle client error only has a few possible error types, to
account for the fact that at this point errors should be returned to the
user.

proxy/src/context/parquet.rs

@@ -108,7 +108,7 @@ impl From<RequestMonitoring> for RequestData {
             branch: value.branch.as_deref().map(String::from),
             protocol: value.protocol,
             region: value.region,
-            error: value.error_kind.as_ref().map(|e| e.to_str()),
+            error: value.error_kind.as_ref().map(|e| e.to_metric_label()),
             success: value.success,
             duration_us: SystemTime::from(value.first_packet)
                 .elapsed()