Redirect gRPC and Protobuf logs into Nginx error log.

By default, these libraries may log directly into stderr.

src/http_module.cpp

@@ -4,6 +4,9 @@ extern "C" {
 #include <ngx_http.h>
 }
 
+#include <grpc/support/log.h>
+#include <google/protobuf/stubs/logging.h>
+
 #include "str_view.hpp"
 #include "trace_context.hpp"
 #include "batch_exporter.hpp"
@@ -475,6 +478,28 @@ ngx_int_t onRequestEnd(ngx_http_request_t* r)
     return NGX_DECLINED;
 }
 
+void grpcLogHandler(gpr_log_func_args* args)
+{
+    ngx_uint_t level = args->severity == GPR_LOG_SEVERITY_ERROR ? NGX_LOG_ERR :
+                       args->severity == GPR_LOG_SEVERITY_INFO  ? NGX_LOG_INFO :
+                                       /*GPR_LOG_SEVERITY_DEBUG*/ NGX_LOG_DEBUG;
+
+    ngx_log_error(level, ngx_cycle->log, 0, "OTel/grpc: %s", args->message);
+}
+
+void protobufLogHandler(google::protobuf::LogLevel logLevel,
+    const char* filename, int line, const std::string& msg)
+{
+    using namespace google::protobuf;
+
+    ngx_uint_t level = logLevel == LOGLEVEL_FATAL   ? NGX_LOG_EMERG :
+                       logLevel == LOGLEVEL_ERROR   ? NGX_LOG_ERR :
+                       logLevel == LOGLEVEL_WARNING ? NGX_LOG_WARN :
+                                 /*LOGLEVEL_INFO*/    NGX_LOG_INFO;
+
+    ngx_log_error(level, ngx_cycle->log, 0, "OTel/protobuf: %s", msg.c_str());
+}
+
 ngx_int_t initModule(ngx_conf_t* cf)
 {
     auto cmcf = (ngx_http_core_main_conf_t*)ngx_http_conf_get_module_main_conf(
@@ -496,6 +521,9 @@ ngx_int_t initModule(ngx_conf_t* cf)
 
     *h = onRequestEnd;
 
+    gpr_set_log_function(grpcLogHandler);
+    google::protobuf::SetLogHandler(protobufLogHandler);
+
     return NGX_OK;
 }