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/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>,