refactor(connlib): use RwLock for feature flags (#8397)

Most of the time, these flags are only read from and not written thus.
By using a read-lock, we make sure that even when we use feature-flags
from multiple threads, they don't cause any contention.

rust/telemetry/src/lib.rs

@@ -7,7 +7,7 @@ use std::{
 
 use anyhow::{Context, Result};
 use env::ON_PREM;
-use parking_lot::Mutex;
+use parking_lot::RwLock;
 use sentry::protocol::SessionStatus;
 use serde::{Deserialize, Serialize};
 
@@ -31,7 +31,7 @@ const POSTHOG_API_KEY: &str = "phc_uXXl56plyvIBHj81WwXBLtdPElIRbm7keRTdUCmk8ll";
 // Process-wide storage of enabled feature flags.
 //
 // Defaults to everything off.
-static FEATURE_FLAGS: LazyLock<Mutex<FeatureFlags>> = LazyLock::new(Mutex::default);
+static FEATURE_FLAGS: LazyLock<RwLock<FeatureFlags>> = LazyLock::new(RwLock::default);
 
 /// Exposes all feature flags as public, static functions.
 ///
@@ -40,7 +40,7 @@ pub mod feature_flags {
     use crate::*;
 
     pub fn icmp_unreachable_instead_of_nat64() -> bool {
-        FEATURE_FLAGS.lock().icmp_unreachable_instead_of_nat64
+        FEATURE_FLAGS.read().icmp_unreachable_instead_of_nat64
     }
 }
 
@@ -188,7 +188,7 @@ impl Telemetry {
 
             tracing::debug!(?flags, "Evaluated feature-flags");
 
-            *FEATURE_FLAGS.lock() = flags;
+            *FEATURE_FLAGS.write() = flags;
 
             sentry::Hub::main().configure_scope(|scope| {
                 scope.set_context("flags", sentry_flag_context(flags));