[receiver-mock] Add pretty printing for `Sample` objects

[aboguszewski-sumo]

The default debug display prints objects in JSON. This type should have fmt::Display trait defined so that the debug logs look better.

Example (and as for now, the only) print:

sumologic-kubernetes-tools/src/rust/receiver-mock/src/router/otlp.rs

Line 140 in 41ace39

println!("metrics => {:?}", m);