[tf.data] Add counters for tf.data elements, autotuning, and optimizations.

This CL:
- adds counters for tf.data elements, autotuning and optimizations
- sets the number of iterations of the `tf_data_meta_optimizer` to one -- the iteration of tf.data optimizations is handled by the tf.data meta optimizer itself
- adds the `alwayslink` attribute to all tf.data optimization BUILD targets to make sure they are always registered (without this, they would not be registered for the Tensorflow server binary I was using for local testing) and further cleans up visibility and dependencies of //third_party/tensorflow/core/grappler/optimizers/data/BUILD
- introduces TFDataOptimizerBase as a base class for tf.data optimizations
- moves TensorFlow metrics into tensorflow::metrics namespace

PiperOrigin-RevId: 229302097