Implement the Value and KeyValue types

[MrAlias]

reference: https://github.com/open-telemetry/opentelemetry-go/pull/4725/files#diff-6176e76e800e6ce0edb44f3d31adaca4602b7cc03cbe54f5ea233a7f2edd17a0

• Value type
  • Kind method
  • AsAny method
  • AsString method
  • AsInt64 method
  • AsBool method
  • AsFloat64 method
  • AsBytes method
  • AsSlice method
  • AsMap method
  • Empty method
  • Equal method
  • StringValue function
  • IntValue function
  • Int64Value function
  • Float64Value function
  • BoolValue function
  • BytesValue function
  • SliceValue function
  • MapValue function
• KeyValue type
  • Equal method
• String function
• Int64 function
• Int function
• Float64 function
• Bool function
• Bytes function
• Slice function
• Map function
• Add tests to cover all added functionality

[MrAlias]

The reference implementation includes panics in many of the Value methods:

• https://github.com/open-telemetry/opentelemetry-go/pull/4725/files#diff-6176e76e800e6ce0edb44f3d31adaca4602b7cc03cbe54f5ea233a7f2edd17a0R151
• https://github.com/open-telemetry/opentelemetry-go/pull/4725/files#diff-6176e76e800e6ce0edb44f3d31adaca4602b7cc03cbe54f5ea233a7f2edd17a0R162
• https://github.com/open-telemetry/opentelemetry-go/pull/4725/files#diff-6176e76e800e6ce0edb44f3d31adaca4602b7cc03cbe54f5ea233a7f2edd17a0R171
• https://github.com/open-telemetry/opentelemetry-go/pull/4725/files#diff-6176e76e800e6ce0edb44f3d31adaca4602b7cc03cbe54f5ea233a7f2edd17a0R184
• https://github.com/open-telemetry/opentelemetry-go/pull/4725/files#diff-6176e76e800e6ce0edb44f3d31adaca4602b7cc03cbe54f5ea233a7f2edd17a0R200
• https://github.com/open-telemetry/opentelemetry-go/pull/4725/files#diff-6176e76e800e6ce0edb44f3d31adaca4602b7cc03cbe54f5ea233a7f2edd17a0R213
• https://github.com/open-telemetry/opentelemetry-go/pull/4725/files#diff-6176e76e800e6ce0edb44f3d31adaca4602b7cc03cbe54f5ea233a7f2edd17a0R226
• https://github.com/open-telemetry/opentelemetry-go/pull/4725/files#diff-6176e76e800e6ce0edb44f3d31adaca4602b7cc03cbe54f5ea233a7f2edd17a0R261
• https://github.com/open-telemetry/opentelemetry-go/pull/4725/files#diff-6176e76e800e6ce0edb44f3d31adaca4602b7cc03cbe54f5ea233a7f2edd17a0R287

Do we want to panic in these cases? This could cause runtime panics for users using OTel and it seems like it might be violating the OTel exception policy.

[MrAlias]

Instead of panicking we could log an error and return a zero value?

Maybe also return a default value in the case of something like func (*Value) String() string which could just use fmt.Sprint?

[MrAlias]

For reference, the existing attribute package does not panic: https://github.com/open-telemetry/opentelemetry-go/blob/main/attribute/value.go

[pellared]

Given the OTel Spec says

API methods MUST NOT throw unhandled exceptions when used incorrectly by end users.

and what the existing attribute package does

I support @MrAlias proposal:

Instead of panicking we could log an error and return a zero value

Note

It would be good describe it in DESIGN.md as e.g. slog panics in equivalent methods.

[pellared]

I updated the prototype (reference implementation): 147762f

EDIT: I also created #4948