outbound: Add logging for endpoint opaqueness (#1614)

The outbound proxy's logging doesn't make it clear how targets decide
whether protocol detection is employed. This change adds debug logging
to make it easier to understand these decisions.

Signed-off-by: Oliver Gould <ver@buoyant.io>

linkerd/app/outbound/src/http/detect.rs

@@ -60,8 +60,10 @@ impl<N> Outbound<N> {
                     // detection and just use the TCP stack directly.
                     |target: T| -> Result<_, Infallible> {
                         if let Some(Skip) = target.param() {
+                            tracing::debug!("Skipping HTTP protocol detection");
                             return Ok(svc::Either::B(target));
                         }
+                        tracing::debug!("Attempting HTTP protocol detection");
                         Ok(svc::Either::A(target))
                     },
                     skipped,

linkerd/app/outbound/src/switch_logical.rs

@@ -35,24 +35,28 @@ impl<S> Outbound<S> {
                             // If the profile provides an endpoint, then the target is single endpoint and
                             // not a logical/load-balanced service.
                             if let Some((addr, metadata)) = rx.endpoint() {
+                                let is_opaque = rx.is_opaque_protocol();
+                                tracing::debug!(%is_opaque, "Profile describes an endpoint");
                                 return Ok(svc::Either::A(Endpoint::from_metadata(
                                     addr,
                                     metadata,
                                     no_tls_reason,
-                                    rx.is_opaque_protocol(),
+                                    is_opaque,
                                     &*inbound_ips,
                                 )));
                             }
 
                             // Otherwise, if the profile provides a (named) logical address, then we build a
                             // logical stack so we apply routes, traffic splits, and load balancing.
                             if let Some(logical_addr) = rx.logical_addr() {
+                                tracing::debug!("Profile describes a logical service");
                                 return Ok(svc::Either::B(Logical::new(logical_addr, rx)));
                             }
                         }
 
                         // If there was no profile or it didn't include any useful metadata, create a bare
                         // endpoint from the original destination address.
+                        tracing::debug!("No profile; forwarding to the original destination");
                         Ok(svc::Either::A(Endpoint::forward(
                             target.param(),
                             no_tls_reason,