Make fileds in `GrpcMethod` public.

## Feature Request

Make `service` and `method` static fields public, like this:

```rust
/// A gRPC Method info extension.
#[derive(Debug, Clone)]
pub struct GrpcMethod {
pub service: &'static str,
pub method: &'static str,
}
```

### Motivation

It's often needed for metrics and dumping messages. Some libraries required `Cow<'static, str>` or even `&'static str` for metric an dumping labels.

### Proposal

A see no drawbacks except increasing public interface surface.

### Alternatives

leave as is.