metrics: Update documentation

The link that points to all known strings is outdated.

Change-Id: I40f7fd2b80fc0129eff5947ed6881d11934cb275
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/1998084
Auto-Submit: Edward Lesmes <ehmaldonado@chromium.org>
Reviewed-by: Anthony Polito <apolito@google.com>
Commit-Queue: Anthony Polito <apolito@google.com>

metrics.README.md

@@ -30,7 +30,7 @@ First, some words about what data we are **NOT** collecting:
 - We won't record information about the current directory or environment flags.
 - We won't record arbitrary strings. We only collect a string if it is in the
   list available at
-  https://chromium.googlesource.com/infra/infra/+/master/go/src/infra/appengine/depot_tools_metrics/metrics/metrics_schema.json#45
+  https://source.chromium.org/chromium/infra/infra/+/master:go/src/infra/appengine/depot_tools_metrics/metrics/constants.go
 
 The metrics we're collecting are:
 
@@ -72,7 +72,10 @@ The metrics we're collecting are:
   - What was the exit code?
 
 The list of all known strings we collect can be found at
-https://chromium.googlesource.com/infra/infra/+/master/go/src/infra/appengine/depot_tools_metrics/metrics/metrics_schema.json#45
+https://source.chromium.org/chromium/infra/infra/+/master:go/src/infra/appengine/depot_tools_metrics/metrics/constants.go
+
+The schema for the metrics being collected can be found at
+https://source.chromium.org/chromium/infra/infra/+/master:go/src/infra/appengine/depot_tools_metrics/schema/schema.proto
 
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