stats: Update documentation and examples to demonstrate enabling `experimental` metrics

[purnesh42H]

There have been several issues opened recently like this asking about not being able to see experimental metrics from wrr and xds. It is because they are not enabled by default and needs to be added to the DefaultMetrics in the dial and server options.

This is the list of all experimental metrics https://grpc.io/docs/guides/opentelemetry-metrics/#instruments. We should include this part in the example and also sample code for adding experimental metrics.

Update documentation and example client and server here https://github.com/grpc/grpc-go/tree/master/examples/features/opentelemetry

[vinothkumarr227]

@purnesh42H I'm looking into this. Please assign the issue to me.

[purnesh42H]

Noticed that we have go doc examples here https://github.com/grpc/grpc-go/blob/master/stats/opentelemetry/example_test.go. We should add one for appending the experimental metrics here as well. We have for removing a metric from DefaultMetrics but not for adding.

Also, the godoc doesn't show all the examples from example_test.go https://pkg.go.dev/google.golang.org/grpc/stats/opentelemetry#pkg-examples. We should update the names as per specifications here https://go.dev/blog/examples

@dfawley @vinothkumarr227 cc