Ref(generic-metrics): set default for index-tag-values to False (#52370)

### Overview

Generic metrics tag values should not be indexed, setting to True as
default leads to local sentry instances indexing tag values.

src/sentry/options/defaults.py

@@ -911,7 +911,7 @@
 # Flag to determine whether performance metrics indexer should index tag
 # values or not
 register(
-    "sentry-metrics.performance.index-tag-values", default=True, flags=FLAG_AUTOMATOR_MODIFIABLE
+    "sentry-metrics.performance.index-tag-values", default=False, flags=FLAG_AUTOMATOR_MODIFIABLE
 )