Replace let _ with let _enter

See discussion here: rust-lang/rust-clippy#8246

tl;dr: `let _` drops immediately, so for anything that has Guard
like behavior, it will not work as expected. `let _enter` will
ensure that the guard is dropped at the end of the scope.

axum-tracing-opentelemetry/src/middleware/trace_extractor.rs

@@ -135,7 +135,7 @@ where
             tracing::Span::none()
         };
         let future = {
-            let _ = span.enter();
+            let _enter = span.enter();
             self.inner.call(req)
         };
         ResponseFuture {

tonic-tracing-opentelemetry/src/middleware/client.rs

@@ -64,7 +64,7 @@ where
         let span = otel_http::grpc_client::make_span_from_request(&req);
         otel_http::inject_context(&find_context_from_tracing(&span), req.headers_mut());
         let future = {
-            let _ = span.enter();
+            let _enter = span.enter();
             self.inner.call(req)
         };
         ResponseFuture {

tonic-tracing-opentelemetry/src/middleware/server.rs

@@ -86,7 +86,7 @@ where
             tracing::Span::none()
         };
         let future = {
-            let _ = span.enter();
+            let _enter = span.enter();
             self.inner.call(req)
         };
         ResponseFuture {