Make built-in metrics enabled by default

This change inverts the logic for enabling built-in OpenTelemetry
metrics. Previously, metrics were disabled by default and could be
enabled by setting `ENABLE_SPANNER_METRICS_ENV_VAR=true`.

With this update, metrics are now enabled by default to provide better
out-of-the-box observability for users.

To disable metrics, users must now set the new environment variable:
`SPANNER_DISABLE_BUILTIN_METRICS=true`

The old `ENABLE_SPANNER_METRICS_ENV_VAR` is no longer used. Unit tests
have been updated to reflect this new opt-out behavior.

**BREAKING CHANGE**: Built-in metrics are now enabled by default. Users
who previously did not set any environment variables will have metrics
collection and export turned on automatically after upgrading. To
restore the previous behavior and disable metrics, thry have to set the
`SPANNER_DISABLE_BUILTIN_METRICS` environment variable to `true`.