System information

• TensorFlow.js version (you are using): Latest, 4.17.0
• Are you willing to contribute it (Yes/No): Yes

Describe the feature and the current behavior/state.
Computing the R2 Score (coeficient of determination)
Currently there is no R2 Score metrics in TFJS compared to TF.

Will this change the current api? How?
Yes. Add's a new metrics in TFJS (tf.metrics.r2Score)
https://js.tensorflow.org/api/latest/#Metrics

Who will benefit with this feature?
Regression Tasks (e.g. Timeseries Forecasting) during training and evaluation.
More convenience in developing (since it's not an own/customer function anymore).

Any Other info.
https://www.tensorflow.org/api_docs/python/tf/keras/metrics/R2Score

Many thanks and looking forward.