refactor(layers/prometheus-client): remove useless scheme field from PrometheusAccessor and PrometheusMetricWrapper type

[koushiro]

Which issue does this PR close?

No

Rationale for this change

simplify the code

What changes are included in this PR?

• remove useless scheme field from PrometheusAccessor and PrometheusMetricWrapper type
• add help for some metrics

Are there any user-facing changes?

No

[koushiro]

@Xuanwo What do you think of removing the opendal_ prefix of these metrics?
Users can customize prefix through the following code:

let builder = services::Memory::default();
let mut registry = prometheus_client::registry::Registry::default();

let op = Operator::new(builder)
        .expect("must init")
        .layer(PrometheusClientLayer::new(registry.sub_registry_with_prefix("opendal")))
        .finish();

[Xuanwo]

I want to retain the opendal_ prefix to clearly indicate that these metrics are emitted by opendal, similar to how alertmanager does.

[koushiro]

But for PrometheusLayer (not PrometheusClientLayer), there is no builtin opendal_ prefix in the metrics.

[koushiro]

Actually, I want the metrics exported by these two layers to be the same, the only difference being the different prometheus libraries they use, but that is not the case now.

[Xuanwo]

I prefer to add prefix for PrometheusLayer

[koushiro]

Ok, I will consider unifying the metrics of these two prometheus layers in a later PR.

[Xuanwo]

Ok, I will consider unifying the metrics of these two prometheus layers in a later PR.

Thanks a lot for the effort!

[Xuanwo]

Great, thanks a lot!