Collect number of connections by sni type (#5867)

## Problem

We don't know the number of users with the different kind of
authentication: ["sni", "endpoint in options" (A and B from
[here](https://neon.tech/docs/connect/connection-errors)),
"password_hack"]

## Summary of changes

Collect metrics by sni kind.

proxy/src/auth/credentials.rs

@@ -1,7 +1,9 @@
 //! User credentials used in authentication.
 
 use crate::{
-    auth::password_hack::parse_endpoint_param, error::UserFacingError, proxy::neon_options,
+    auth::password_hack::parse_endpoint_param,
+    error::UserFacingError,
+    proxy::{neon_options, NUM_CONNECTION_ACCEPTED_BY_SNI},
 };
 use itertools::Itertools;
 use pq_proto::StartupMessageParams;
@@ -124,6 +126,22 @@ impl<'a> ClientCredentials<'a> {
         .transpose()?;
 
         info!(user, project = project.as_deref(), "credentials");
+        if sni.is_some() {
+            info!("Connection with sni");
+            NUM_CONNECTION_ACCEPTED_BY_SNI
+                .with_label_values(&["sni"])
+                .inc();
+        } else if project.is_some() {
+            NUM_CONNECTION_ACCEPTED_BY_SNI
+                .with_label_values(&["no_sni"])
+                .inc();
+            info!("Connection without sni");
+        } else {
+            NUM_CONNECTION_ACCEPTED_BY_SNI
+                .with_label_values(&["password_hack"])
+                .inc();
+            info!("Connection with password hack");
+        }
 
         let cache_key = format!(
             "{}{}",

proxy/src/proxy.rs

@@ -129,6 +129,15 @@ pub static RATE_LIMITER_LIMIT: Lazy<IntGaugeVec> = Lazy::new(|| {
     .unwrap()
 });
 
+pub static NUM_CONNECTION_ACCEPTED_BY_SNI: Lazy<IntCounterVec> = Lazy::new(|| {
+    register_int_counter_vec!(
+        "proxy_accepted_connections_by_sni",
+        "Number of connections (per sni).",
+        &["kind"],
+    )
+    .unwrap()
+});
+
 pub struct LatencyTimer {
     // time since the stopwatch was started
     start: Option<Instant>,