Add `#[flatten]` directive for derivation of nested structs

[howardjohn]

Given the structs like:

struct Common {
  field_a: String,
  field_b: String,
  field_c: String
}

#[derive(Encode)]
struct Metric1 {
  unique_field: String
  #[flatten]
  common: Common
}

#[derive(Encode)]
struct Metric2 {
  unique_field_2: String
  #[serde(flatten)]
  common: Common
}

I want the resulting metrics:

metric1{field_a="foo", field_b="bar", field_c="baz",unique_field="something"}
metric2{field_a="foo", field_b="bar", field_c="baz",unique_field_2="something_else"}

(Note: in real world Common and the number of metrics are much larger)

This would match the semantics of serde(flatten)

See #105 (comment) for more discussion

[mxinden]

I have had that same use-case in rust-libp2p before. I am in favor of adding this to prometheus-client-derive-encode.

I foresee having the capacity to implement this any time soon. Focusing all my capacity on #74. @howardjohn are you interested in making the contribution?

[howardjohn]

@mxinden I sent a PR in #118. it might need some improvements but it works in the basic case right now, at least.