Name of a feature flag must contain at least 2 characters. Can only
contain lowercase letters, digits, and '_'. Must start with a letter,
and cannot end with '_'.

Feature flag name would be used to construct the corresponding metadata
key, so:

- Only characters allowed by grpc metadata keys can be used and
  uppercase letters would be normalized to lower, see
  https://pkg.go.dev/google.golang.org/grpc/metadata#New
- It is critical that feature flags don't contain a dash, because the
  client converts dashes to underscores when converting a feature flag's
  name to the metadata key, and vice versa. The name wouldn't round-trip
  in case it had underscores and must thus use dashes instead.

Signed-off-by: blanet <moweng.xx@alibaba-inc.com>
Helped-by: John Cai <jcai@gitlab.com>
Changelog: fixed