clean up exrmetrics help, simplify JSON output (#1973)

* clean up exrmetrics help, simplify JSON output

Signed-off-by: Peter Hillman <peterh@wetafx.co.nz>

* revert exrmetrics to returncode 0 with --help

Signed-off-by: Peter Hillman <peterh@wetafx.co.nz>

---------

Signed-off-by: Peter Hillman <peterh@wetafx.co.nz>

Author: peterhillman

src/bin/exrmetrics/main.cpp

@@ -59,13 +59,13 @@ usageMessage (ostream& stream, const char* program_name, bool verbose = false)
                "                              default is \"all\" \n"
                "\n"
                "  -m                          set to multi-threaded (system selected thread count)\n"
-               "  -t n                        use n threads for processing files\n"
-               "                              default is single / no threads\n"
+               "  -t n                        Use a pool of n worker threads for processing files.\n"
+               "                              Default is single threaded (no thread pool)\n"
                "\n"
                "  -l level                    set DWA or ZIP compression level\n"
                "\n"
                "  -z,--compression list       list of compression methods to test\n"
-               "                ("
+               "                              ("
             << compressionNames.c_str ()
             << ",orig,all\n"
                "                              default orig: retains original method)\n"
@@ -299,7 +299,7 @@ jsonStats (
                         << run.metrics.totalStats.sizeData.tileCount << ",\n";
                 }
 
-                if (run.metrics.stats.size () > 0)
+                if (run.metrics.stats.size () > 1)
                 {
                     out << "  \"parts\":\n";
                     out << "   [\n";
@@ -482,6 +482,8 @@ main (int argc, char** argv)
 
     if (opts.parse (argc, argv)) { return 1; }
 
+    if (opts.inFiles.size () == 0) { return 0; }
+
     list<runData> data;
     try
     {