WatchOS availability in my country

If you are talking about ECG, then that is dependent on your countries health agencies giving Apple approval to enable the ECG app in your country.

If you are talking about cellular carriers that is partially up to the cellular carriers in your country

Cellular Carriers that support the Apple Watch, which models, the radio frequencies each model uses, and the country they offer service:

https://www.apple.com/watch/cellular

As for other features, here is a list of Apple Watch Feature Availability by country

https://www.apple.com/watchos/feature-availability

I couldn't find any of the extra features listed for Sri Lanka. This is a User to User forum and we have no idea when or if Apple will have these features available or if your country would approve of them.