fix: gate protocol validation on transport features for clippy

The protocol validation block in build_channel uses resolve_protocol
and Protocol which are only available with grpc-tonic/http-proto/http-json
features. Gate the block to fix clippy with experimental-grpc-retry only.

Author: bryantbiggs

opentelemetry-otlp/src/exporter/tonic/mod.rs

@@ -19,9 +19,7 @@ use tonic::transport::ClientTlsConfig;
 use super::{default_headers, parse_header_string, OTEL_EXPORTER_OTLP_GRPC_ENDPOINT_DEFAULT};
 use super::{resolve_timeout, ExporterBuildError};
 use crate::exporter::Compression;
-use crate::{
-    exporter::ExportConfig, Protocol, OTEL_EXPORTER_OTLP_ENDPOINT, OTEL_EXPORTER_OTLP_HEADERS,
-};
+use crate::{exporter::ExportConfig, OTEL_EXPORTER_OTLP_ENDPOINT, OTEL_EXPORTER_OTLP_HEADERS};
 
 #[cfg(all(
     feature = "experimental-grpc-retry",
@@ -209,18 +207,26 @@ impl TonicExporterBuilder {
         // Env var-based transport selection is handled at the auto-select layer
         // (e.g., SpanExporter::builder().build()), which routes to the correct
         // transport before reaching this code.
-        let protocol = super::resolve_protocol(signal_protocol_var, self.exporter_config.protocol);
-
-        let is_http_protocol = false;
-        #[cfg(feature = "http-proto")]
-        let is_http_protocol = is_http_protocol || matches!(protocol, Protocol::HttpBinary);
-        #[cfg(feature = "http-json")]
-        let is_http_protocol = is_http_protocol || matches!(protocol, Protocol::HttpJson);
-        if is_http_protocol {
-            return Err(ExporterBuildError::InvalidConfig {
-                name: "protocol".to_string(),
-                reason: "HTTP protocol is not compatible with gRPC transport. Use `.with_http()` instead.".to_string(),
-            });
+        #[cfg(any(feature = "grpc-tonic", feature = "http-proto", feature = "http-json"))]
+        {
+            let protocol =
+                super::resolve_protocol(signal_protocol_var, self.exporter_config.protocol);
+
+            let is_http_protocol = false;
+            #[cfg(feature = "http-proto")]
+            let is_http_protocol =
+                is_http_protocol || matches!(protocol, crate::Protocol::HttpBinary);
+            #[cfg(feature = "http-json")]
+            let is_http_protocol =
+                is_http_protocol || matches!(protocol, crate::Protocol::HttpJson);
+            if is_http_protocol {
+                return Err(ExporterBuildError::InvalidConfig {
+                    name: "protocol".to_string(),
+                    reason:
+                        "HTTP protocol is not compatible with gRPC transport. Use `.with_http()` instead."
+                            .to_string(),
+                });
+            }
         }
 
         let compression = self.resolve_compression(signal_compression_var)?;