Deprecate the Greentea metrics API

Add the deprecated macro to where the Greentea metrics API is declared.

Fixes #14790

Author: nicole-osaz

features/frameworks/greentea-client/greentea-client/greentea_metrics.h

@@ -7,11 +7,13 @@
 /**
  *  Setup platform specific metrics
  */
+MBED_DEPRECATED_SINCE("mbed-os-6.13", "Greentea metrics API are deprecated")
 void greentea_metrics_setup(void);
 
 /**
  *  Report and cleanup platform specifc metrics
  */
+MBED_DEPRECATED_SINCE("mbed-os-6.13", "Greentea metrics API are deprecated")
 void greentea_metrics_report(void);
 
 #endif