getMetadata() for Session is not handling the promise

[alkatrivedi]

Unable to get the metadata for a Session using await instead of callback.

Error: TypeError: callback is not a function at callback!(err, resp);

Steps to reproduce -

const {Spanner} = require('@google-cloud/spanner');
const spanner = new Spanner({
  projectId: projectId,
});
const instance = spanner.instance(instanceId);
const database = instance.database(databaseId);
const session = await database.createSession();
const metadata = await session[0].getMetadata();