redis: db.namespace is not shown for Redis database index in stable semconv mode

Describe your environment

OS: Mac OS 26.6.2
Python version: 3.12
Package version: 0.66b0.dev

What happened?

The Redis database index is shown only in the default database semconv mode. When stable semconv mode is enabled, the database index is not shown.

In _set_db_redis_database_index, the comment says that db.redis.database_index was removed with no replacement in semconv 1.38.0. However, db.redis.database_index was replaced by db.namespace in semconv 1.26.0

Steps to Reproduce

https://github.com/open-telemetry/opentelemetry-python-contrib/blob/466ae4c00de77d1f54f77a9600f1d08eba2306b5/opentelemetry-instrumentation/src/opentelemetry/instrumentation/_semconv.py#L605-L613

Expected Result

Default semconv mode should show db.redis.database_index and stable semconv mode should show db.namespace. In database/dup, both db.redis.database_index and db.namespace should be shown.

Actual Result

db.redis.database_index is shown in default semconv and db.namespace is not shown in stable semconv.

Additional context

No response

Would you like to implement a fix?

None

Tip

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Start in opentelemetry-instrumentation/src/opentelemetry/instrumentation/_semconv.py at _set_db_redis_database_index and review the surrounding semconv-mode handling. Reproduce the Redis database index behavior in default, stable, and database/dup modes, then verify that each mode emits the attributes described in the expected result.