Support publishing multiple metrics with different dimensions

[yaozhaoy]

Due to latency concerns, customers want to be able to add metrics with different dimensions before call flush().

Runtime: Lambda

[thisissoumyasaha]

Thanks. Any updates about ETA on this?

[thisissoumyasaha]

Also, I saw a TODO about async flush. Is that an option are you pursuing?

[kristofvb]

I just realized that my pull request is probably asking for the same thing.