[otap-dataflow] Add Sampling (based on zap) to DebugProcessor (open-telemetry#1250)

Added the SamplingConfig struct to allow the user to configure their
sampling settings

```rust
#[derive(Debug, Clone, Copy, Deserialize)]
pub struct SamplingConfig {
    // number of samples to log initially within the sampling interval
    pub sampling_initial: u64,
    // the sampling rate
    pub sampling_thereafter: u64,
    // the interval that we sample in, unit is seconds
    pub sampling_interval: u64,
}
```

Added the Sampler struct to handle the sampling logic for the debug
processor

```rust
/// The sampler keeps track of the current state, number of msgs seen and next interval
pub struct Sampler {
    // sampling settings
    sampling_config: SamplingConfig,
    // counter for msgs seen
    msgs_current_interval: u64,
    next_interval: Instant,
}
```

uses the sample function that accepts a closure (in this closure is the
call to output the msgs either to console or outport depending on
configuration) to determine whether to log or drop a msg

Author: c1ly