[llvm] Document that llvm-lit needs to be run to collect static metrics (PR #94836)

[via llvm-commits]

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@@ -266,7 +269,7 @@ Example usage:
 
   ```bash
   % test-suite/utils/compare.py -m compile_time baseline.json
-  % test-suite/utils/compare.py -m size.__text baseline.json
+  % test-suite/utils/compare.py -m size..text baseline.json
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hiraditya wrote:

i see. i'll remove this.

https://github.com/llvm/llvm-project/pull/94836