metrics: add missing C++ guards

[ upstream commit 4e90b3a ]

Some public header files were missing 'extern "C"' C++ guards,
and couldn't be used by C++ applications. Add the missing guards.

Fixes: c5b7197 ("telemetry: move some functions to metrics library")

Signed-off-by: Brian Dooley <brian.dooley@intel.com>
Acked-by: Bruce Richardson <bruce.richardson@intel.com>
Acked-by: Tyler Retzlaff <roretzla@linux.microsoft.com>

lib/librte_metrics/rte_metrics_telemetry.h

@@ -13,6 +13,9 @@
 #ifndef _RTE_METRICS_TELEMETRY_H_
 #define _RTE_METRICS_TELEMETRY_H_
 
+#ifdef __cplusplus
+extern "C" {
+#endif
 
 enum rte_telemetry_stats_type {
 	PORT_STATS = 0,
@@ -60,4 +63,8 @@ __rte_experimental
 int32_t
 rte_metrics_tel_extract_data(struct telemetry_encode_param *ep, json_t *data);
 
+#ifdef __cplusplus
+}
+#endif
+
 #endif