prevent stats printing from interfering with cli proxy

This makes using stats with the mux possible again:

```
periodic_stat_logging = 10
```

src/frontend/gui/mod.rs

@@ -22,9 +22,7 @@ pub struct GuiFrontEnd {
     connection: Rc<Connection>,
 }
 
-lazy_static::lazy_static! {
-static ref USE_OPENGL: AtomicBool = AtomicBool::new(true);
-}
+static USE_OPENGL: AtomicBool = AtomicBool::new(true);
 
 pub fn is_opengl_enabled() -> bool {
     USE_OPENGL.load(Ordering::Acquire)

src/main.rs

@@ -792,6 +792,8 @@ fn run() -> anyhow::Result<()> {
                     // ourselves into basically netcat.
                     drop(client);
 
+                    crate::stats::disable_stats_printing();
+
                     let front_end = FrontEndSelection::Null.try_new()?;
                     let mux = Rc::new(mux::Mux::new(None));
                     Mux::set_mux(&mux);

src/stats.rs

@@ -2,10 +2,13 @@ use crate::config::configuration;
 use hdrhistogram::Histogram;
 use metrics::{Key, Recorder};
 use std::collections::HashMap;
+use std::sync::atomic::{AtomicBool, Ordering};
 use std::sync::{Arc, Mutex};
 use std::time::{Duration, Instant};
 use tabout::{tabulate_output, Alignment, Column};
 
+static ENABLE_STAT_PRINT: AtomicBool = AtomicBool::new(true);
+
 struct Inner {
     histograms: HashMap<Key, Histogram<u64>>,
 }
@@ -14,6 +17,12 @@ fn pctile_latency(histogram: &Histogram<u64>, p: f64) -> Duration {
     Duration::from_nanos(histogram.value_at_percentile(p))
 }
 
+/// Used to prevent the stats thread from trying to write to stderr
+/// when we're running in proxy mode
+pub fn disable_stats_printing() {
+    ENABLE_STAT_PRINT.store(false, Ordering::Acquire);
+}
+
 impl Inner {
     fn run(inner: Arc<Mutex<Inner>>) {
         let mut last_print = Instant::now();
@@ -39,6 +48,10 @@ impl Inner {
 
         loop {
             std::thread::sleep(Duration::from_secs(10));
+            if !ENABLE_STAT_PRINT.load(Ordering::Acquire) {
+                break;
+            }
+
             let seconds = configuration().periodic_stat_logging;
             if seconds == 0 {
                 continue;