docs(batcher): document how to configure custom aggregators #989 (#1100)
docs(batcher): document how to configure custom aggregators #989 (#1100)

* docs(batcher): document how to configure custom aggregators #989

* chore: address PR comments

Co-authored-by: Mayur Kale <7b800b27b32ee7e0fdb93e2c1dbfb3464ab1d5e6@google.com>
open-telemetry/opentelemetry-js,open-telemetry/opentelemetry-js,open-telemetry/opentelemetry-js,open-telemetry/opentelemetry-js