Improve documentation and tooling around integration testing of services

## Feature Request

### Crates

`tonic`

### Motivation

It is currently a bit unclear to me how client-server should be tested in integration. E.g. what should an implementation of

```rust
fn test_helper(message: FooRequest, expected: Vec)
```

be to test a specific server-client of one of the services?

### Proposal

Add an example (and potentially helper methods) to make it easy to write tests.

### Alternatives

Leave it as is