fix(derive-encode): Cumulative fixes

[ADD-SP]

Summary

Better to rebase and merge to keep the history.

• keep structs generic parameters while while deriving EncodeLabelSet.
• Emit better compilation error message.
• Add missing changelog entry for fix(derive-encode): expand prelude symbols to absolute paths #265

Better to rebase and merge to keep the history

Keep struct generic parameters (#268)

struct CpuUsageLabelsSet<'a> {
    mode: CpuMode,   // <- an enum
    service: &'a str,
}

Currently, the derive macro EncodeLabelSet doesn't keep the generics in the impl block.

Actually, all generics of LabelSet will be propagated to the struct Family,
which makes concurrency programming much harder.

However, it is also usual to use non-shared Family struct in applications,
introducing lifetime on LabelSet eliminates extra memory copies,
which could improve the performance.

To be honest, this might not shows significant improvement, so I considered it as a fix instead of perf, but this gives the freedom of utilizing lifetime to the end developer.

Emit better compilation error message (#269)

// ========== before ==========
error: proc-macro derive panicked
 --> tests/build/friendly-compilation-error-msg.rs:7:49
  |
7 |     #[derive(Debug, Clone, PartialEq, Eq, Hash, EncodeLabelSet)]
  |                                                 ^^^^^^^^^^^^^^
  |
  = help: message: Can not derive Encode for struct with unnamed fields.
// ========== before ==========

// ========== after==========
error: Can not derive `EncodeLabelSet` for struct with unnamed fields.
 --> tests/build/friendly-compilation-error-msg.rs:8:12
  |
8 |     struct Unnamed(String);
  |            ^^^^^^^
// ========== after==========

Issues

closes #268, closes #269