gnomon: add page (#2472)

pages/common/gnomon.md

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+# gnomon 
+
+> Utility to annotate console logging statements with timestamps and find slow processes.
+
+- Use UNIX (or DOS) pipes to pipe the stdout of any command through gnomon:
+
+`{{npm test}} | gnomon`
+
+- Show number of seconds since the start of the process:
+
+`{{npm test}} | gnomon --type=elapsed-total`
+
+- Show an absolute timestamp in UTC:
+
+`{{npm test}} | gnomon --type=absolute`
+
+- Set a high threshold of 0.5 seconds for the elapsed time; exceeding which the timestamp will be colored bright red:
+
+`{{npm test}} | gnomon --high {{0.5}}`
+
+- Set a medium threshold of 0.2 seconds (Timestamp will be colored bright yellow):
+
+`{{npm test}} | gnomon --medium {{0.2}}`