Add missing ext declaration for log detail::format (#1482)

The `format` function is used by [debug and trace loggers](https://github.com/rapidsai/raft/blob/a44ca96c5cddec7ca67b510f3c21163d3958bc7e/cpp/include/raft/core/logger-macros.hpp#L44-L75). While PR #1469 has restructured the logger headers it was forgotten to expose `detail::format` in case the `RAFT_EXPLICIT_INSTANTIATE_ONLY` is defined. This PR fixes that.

Authors:
  - Tamas Bela Feher (https://github.com/tfeher)
  - Corey J. Nolet (https://github.com/cjnolet)

Approvers:
  - Allard Hendriksen (https://github.com/ahendriksen)
  - Corey J. Nolet (https://github.com/cjnolet)

URL: #1482

cpp/include/raft/core/logger-ext.hpp

@@ -15,15 +15,19 @@
  */
 #pragma once
 
-#include <memory>         // std::unique_ptr
-#include <string>         // std::string
-#include <unordered_map>  // std::unordered_map
+#include <memory>                       // std::unique_ptr
+#include <raft/core/detail/macros.hpp>  // RAFT_INLINE_CONDITIONAL
+#include <string>                       // std::string
+#include <unordered_map>                // std::unordered_map
 
 namespace raft {
 
 static const std::string RAFT_NAME = "raft";
 static const std::string default_log_pattern("[%L] [%H:%M:%S.%f] %v");
 
+namespace detail {
+RAFT_INLINE_CONDITIONAL std::string format(const char* fmt, ...);
+}
 /**
  * @brief The main Logging class for raft library.
  *

cpp/include/raft/core/logger-inl.hpp

@@ -54,7 +54,7 @@ inline std::string format(const char* fmt, va_list& vl)
   return std::string(buf.data());
 }
 
-inline std::string format(const char* fmt, ...)
+RAFT_INLINE_CONDITIONAL std::string format(const char* fmt, ...)
 {
   va_list vl;
   va_start(vl, fmt);

cpp/test/core/logger.cpp

@@ -14,6 +14,11 @@
  * limitations under the License.
  */
 
+// We set RAFT_ACTIVE_LEVEL to a value that would enable testing trace and debug logs
+// (otherwise trace and debug logs are desabled by default).
+#undef RAFT_ACTIVE_LEVEL
+#define RAFT_ACTIVE_LEVEL 6
+
 #include <gtest/gtest.h>
 #include <raft/core/logger.hpp>
 #include <string>