Metrics: Document customization options

[pdudits]

OpenTelemetry SDK offers very strong tool in form of Aggregation views and customizations as documented e. g. in https://opentelemetry.io/docs/instrumentation/java/manual/#views.

Current spec gives access to this configuration, however in quite roundabout way of registering an AutoConfigurationCustomizerProvider, and in it invoking configurationCustomizer.addMeterProviderCustomizer((builder, config) -> builder.registerView(...)).

An example of achieving that as well as some motivation (adding or removing dimensions of measurement) should be provided in the spec.

[Emily-Jiang]

Do this spec need to clarify this or a blog should be published to share how to customize metrics? @pdudits

[pdudits]

A motivation / use case section would be fine, definetely not a normative part of spec.

[Emily-Jiang]

A motivation / use case section would be fine, definetely not a normative part of spec.

@pdudits would you like to do a PR on this?

[pdudits]

I'll try, I need to learn about a bit more first though 😁