refactor(tests): feature gate frame size histogram assertions

see:

* prometheus/client_rust#242
* prometheus/client_rust#241

for now, refactor this test so that it gates all use of the (proposed)
`sum()` and `count()` accessors behind a temporary feature gate.

Signed-off-by: katelyn martin <[email protected]>

linkerd/app/outbound/src/http/logical/policy/route/backend/metrics/tests.rs

@@ -155,13 +155,21 @@ async fn body_data_layer_records_frames() -> Result<(), Error> {
     tracing::info!("acquiring response body metrics");
     let labels = labels::RouteBackend(parent_ref.clone(), route_ref.clone(), backend_ref.clone());
     let BodyDataMetrics {
-        frames_total,
-        frames_bytes,
+        // TODO(kate): currently, histograms do not expose their observation count or sum. so,
+        // we're left unable to exercise these metrics until prometheus/client_rust#242 lands.
+        //   - https://github.com/prometheus/client_rust/pull/241
+        //   - https://github.com/prometheus/client_rust/pull/242
+        #[cfg(feature = "prometheus-client-rust-242")]
+        frame_size,
+        ..
     } = metrics.get_response_body_metrics(&labels);
 
     // Before we've sent a response, the counter should be zero.
-    assert_eq!(frames_total.get(), 0);
-    assert_eq!(frames_bytes.get(), 0);
+    #[cfg(feature = "prometheus-client-rust-242")]
+    {
+        assert_eq!(frame_size.count(), 0);
+        assert_eq!(frame_size.sum(), 0);
+    }
 
     // Create a response whose body is backed by a channel that we can send chunks to, send it.
     tracing::info!("sending response");
@@ -177,8 +185,11 @@ async fn body_data_layer_records_frames() -> Result<(), Error> {
     };
 
     // Before we've sent any bytes, the counter should be zero.
-    assert_eq!(frames_total.get(), 0);
-    assert_eq!(frames_bytes.get(), 0);
+    #[cfg(feature = "prometheus-client-rust-242")]
+    {
+        assert_eq!(frame_size.count(), 0);
+        assert_eq!(frame_size.sum(), 0);
+    }
 
     // On the client end, poll our call future and await the response.
     tracing::info!("polling service future");
@@ -205,8 +216,10 @@ async fn body_data_layer_records_frames() -> Result<(), Error> {
         resp_tx.send_data("hello".into()).await?;
         let chunk = read_chunk(&mut body).await?;
         debug_assert_eq!("hello".as_bytes(), chunk, "should get same value back out");
-        assert_eq!(frames_total.get(), 1);
-        assert_eq!(frames_bytes.get(), 5);
+        #[cfg(feature = "prometheus-client-rust-242")]
+        assert_eq!(frame_size.count(), 1);
+        #[cfg(feature = "prometheus-client-rust-242")]
+        assert_eq!(frame_size.sum(), 5);
     }
 
     {
@@ -219,8 +232,10 @@ async fn body_data_layer_records_frames() -> Result<(), Error> {
             chunk,
             "should get same value back out"
         );
-        assert_eq!(frames_total.get(), 2);
-        assert_eq!(frames_bytes.get(), 5 + 8);
+        #[cfg(feature = "prometheus-client-rust-242")]
+        assert_eq!(frame_size.count(), 2);
+        #[cfg(feature = "prometheus-client-rust-242")]
+        assert_eq!(frame_size.sum(), 5 + 8);
     }
 
     {
@@ -233,8 +248,10 @@ async fn body_data_layer_records_frames() -> Result<(), Error> {
             Poll::Ready(None) => {}
             _ => panic!("got unexpected poll result"),
         };
-        assert_eq!(frames_total.get(), 2);
-        assert_eq!(frames_bytes.get(), 5 + 8);
+        #[cfg(feature = "prometheus-client-rust-242")]
+        assert_eq!(frame_size.count(), 2);
+        #[cfg(feature = "prometheus-client-rust-242")]
+        assert_eq!(frame_size.sum(), 5 + 8);
     }
 
     Ok(())