Allow implementations to export log records with duplicate keys in a …

…map as opt-in (open-telemetry#3987)

Fixes open-telemetry#3931

Per agreement: open-telemetry#3931 (comment)

> The SDKs should handle the key-value deduplication by default. It is acceptable to add an option to disable deduplication.

Previous PR: open-telemetry#3938


> I think it is fine to do the deduplication anywhere you want as long as externally observable data complies with this document.

The main purpose of this PR is to have an agreement for following questions (and update the specification to to make it more clear):
1. Is the deduplication required for all log exporters or only OTLP log exporters? Answer: It is required for all exporters.
2. Can the key-value deduplication for log records be opt-in? Answer: Yes, it is OK as long as it is documented that it can cause problems in case maps duplicated keys are exported.

Related to:
- open-telemetry/opentelemetry-go#5086
- open-telemetry/opentelemetry-dotnet#4324

CHANGELOG.md

@@ -22,6 +22,9 @@ release.
 
 ### Logs
 
+- Allow implementations to export duplicate keys in a map as an opt-in option.
+  ([#3938](https://github.com/open-telemetry/opentelemetry-specification/pull/3938))
+
 ### Events
 
 ### Resource

specification/logs/data-model.md

@@ -119,12 +119,21 @@ Value of type `any` can be one of the following:
 #### Type `map<string, any>`
 
 Value of type `map<string, any>` is a map of string keys to `any` values. The
-keys in the map are unique (duplicate keys are not allowed). The representation
-of the map is language-dependent.
+keys in the map are unique (duplicate keys are not allowed).
 
 Arbitrary deep nesting of values for arrays and maps is allowed (essentially
 allows to represent an equivalent of a JSON object).
 
+The representation of the map is language-dependent.
+
+The implementation MUST by default ensure that the exported maps contain only unique keys.
+
+The implementation MAY have an option to allow exporting maps with duplicate keys
+(e.g. for better performance).
+If such option is provided, it MUST be documented that for many receivers,
+handling of maps with duplicate keys is unpredictable and it is the users'
+responsibility to ensure keys are not duplicate.
+
 ### Field Kinds
 
 This Data Model defines a logical model for a log record (irrespective of the