Document that it's ok to "delete" histogram metadata to correct typos

R=asvitkine

Fixed: 1107465
Change-Id: I1465acf2973c177b4224a8a301fa4faf0801b8b4
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2311657Reviewed-by: Alexei Svitkine <asvitkine@chromium.org>
Commit-Queue: Alexei Svitkine <asvitkine@chromium.org>
Auto-Submit: Ilya Sherman <isherman@chromium.org>
Cr-Commit-Position: refs/heads/master@{#791031}

tools/metrics/histograms/README.md

@@ -535,6 +535,10 @@ coming in.  It's also useful to keep obsolete histogram descriptions in
 histogram to answer a particular question, they can learn if there was a
 histogram at some point that did so even if it isn't active now.
 
+*Exception:* It is ok to delete the metadata for any histogram that has never
+been recorded to. For example, it's fine to correct a typo where the histogram
+name in the metadata does not match the name in the Chromium source code.
+
 ### Histogram Suffixes
 
 It is sometimes useful to record several closely related metrics, which measure