Use AccessorInfo instead of seperate fields

[Xuanwo]

Our current obsever::MetricLayer is using seperate fields like:

fn observe_operation_duration_seconds(
    &self,
    scheme: Scheme,
    namespace: Arc<String>,
    root: Arc<String>,
    path: &str,
    op: Operation,
    duration: Duration,
);

This makes it difficult to expand and requires adding a lot of duplicated code for users to call and implement. Also, this will add extra copies of root and name.

We can scope them into Arc<AccessorInfo> instead.