but not able to get Java code level metrics at function level

I don't think function level data is available ... AFAIK opentelemetry get data of standard client libraries used in the application. Have a look at https://github.com/open-telemetry/opentelemetry-java-instrumentation and https://github.com/open-telemetry/opentelemetry-java-instrumentation/blob/main/docs/supported-libraries.md