proxy: improve error classification (#6841)

## Problem

## Summary of changes

1. Classify further cplane API errors
2. add 'serviceratelimit' and make a few of the timeout errors return
that.
3. a few additional minor changes

proxy/src/error.rs

@@ -37,9 +37,12 @@ pub enum ErrorKind {
     /// Network error between user and proxy. Not necessarily user error
     ClientDisconnect,
 
-    /// Proxy self-imposed rate limits
+    /// Proxy self-imposed user rate limits
     RateLimit,
 
+    /// Proxy self-imposed service-wise rate limits
+    ServiceRateLimit,
+
     /// internal errors
     Service,
 
@@ -54,25 +57,12 @@ pub enum ErrorKind {
 }
 
 impl ErrorKind {
-    pub fn to_str(&self) -> &'static str {
-        match self {
-            ErrorKind::User => "request failed due to user error",
-            ErrorKind::ClientDisconnect => "client disconnected",
-            ErrorKind::RateLimit => "request cancelled due to rate limit",
-            ErrorKind::Service => "internal service error",
-            ErrorKind::ControlPlane => "non-retryable control plane error",
-            ErrorKind::Postgres => "postgres error",
-            ErrorKind::Compute => {
-                "non-retryable compute connection error (or exhausted retry capacity)"
-            }
-        }
-    }
-
     pub fn to_metric_label(&self) -> &'static str {
         match self {
             ErrorKind::User => "user",
             ErrorKind::ClientDisconnect => "clientdisconnect",
             ErrorKind::RateLimit => "ratelimit",
+            ErrorKind::ServiceRateLimit => "serviceratelimit",
             ErrorKind::Service => "service",
             ErrorKind::ControlPlane => "controlplane",
             ErrorKind::Postgres => "postgres",
@@ -85,12 +75,6 @@ pub trait ReportableError: fmt::Display + Send + 'static {
     fn get_error_kind(&self) -> ErrorKind;
 }
 
-impl ReportableError for tokio::time::error::Elapsed {
-    fn get_error_kind(&self) -> ErrorKind {
-        ErrorKind::RateLimit
-    }
-}
-
 impl ReportableError for tokio_postgres::error::Error {
     fn get_error_kind(&self) -> ErrorKind {
         if self.as_db_error().is_some() {