Add log messages for empty environment_variable and process_argument values in template variable

Author: mtoffl01

internal-api/src/main/java/datadog/trace/bootstrap/config/provider/StableConfigParser.java

@@ -244,6 +244,8 @@ private static String processTemplateVar(String templateVar) throws IOException
       }
       String value = System.getenv(envVar.toUpperCase());
       if (value == null || value.isEmpty()) {
+        log.debug(
+            "Environment variable specified in template {} has no value", envVar.toUpperCase());
         return UNDEFINED_VALUE;
       }
       return value;
@@ -261,6 +263,7 @@ private static String processTemplateVar(String templateVar) throws IOException
       }
       String value = System.getProperty(processArg.substring(2));
       if (value == null || value.isEmpty()) {
+        log.debug("Process argument specified in template {} has no value", processArg);
         return UNDEFINED_VALUE;
       }
       return value;