Refresh key value collection based on page etag

[zhiyuanliang-ms]

Why this PR?

This PR supports refreshing key value based on page etag (aka key value collection monitoring).

Visible change

• When refreshOption.enabled is true and no watched setting is specified, then the provider will watch all selected key value to refresh the configuration

• This PR also re-organize the AzureAppConfigurationImpl (1. adjust the method order 2. add more comments 3. simplify the code path 4. make more function resuable

Note

Some corner cases:

1. If you load the following kvs and ffs:

{
  selector: [{keyFilter: "*"}],
  refreshOptions: {
    enabled: true
  }
}

with this load option, the feature_management section will not appear in the local configuration (provider will ignore feature flag configuration settings), because user doesn't use the feature flag in the load options (i.e. AzureAppConfigurationOptions.feaetureFlagOptions.

However, if users changes any feature flag through the portal, the page etag of monitored key value collection (of selector with keyFilter of "*") will change. So it will trigger a real refresh. Even if, all the key value loaded to local configuration will be the same as the previous one.

This behavior may only affect the scenario when onRefresh callback is used. Otherwise, user will never notice this.

Reference

.NET provider PR

[RichardChen820]

LGTM