[Metrics] fix help message and enforce test oracle

src/plugins/metrics/metrics_parameters.ml

@@ -49,9 +49,8 @@ module OutputFile =
       let option_name = "-metrics-output"
       let arg_name = "filename"
       let help = "print some metrics into the specified file; \
-                  the output format is recognized through the extension. \
-                  If the filename is only the extension (txt, html, json), \
-                  print to stdout but in the specified format."
+                  the output format is recognized through the extension: \
+                  .text/.txt for text, .html/.htm for HTML, or .json for JSON."
      end)
 
 module ValueCoverage =

tests/metrics/libc.c

 /* run.config
    STDOPT: #"-metrics-no-libc -metrics-eva-cover"
    STDOPT: #"-metrics-libc -metrics-eva-cover"
-   STDOPT: #"-metrics-libc -metrics-output @PTEST_DIR@/oracle/libc.json"
-   LOG: @PTEST_DIR@/oracle/libc.json
+   STDOPT: #"-metrics-libc -metrics-output @PTEST_DIR@/result/libc.json"
+   LOG: libc.json
+   OPT:
 */
 #include <ctype.h>
 #include <stdio.h> // defines external variables

tests/metrics/oracle/libc.3.res.oracle

+[kernel] Parsing tests/metrics/libc.c (with preprocessing)