Specify how to handle instrument name conflicts

CHANGELOG.md

@@ -13,6 +13,9 @@ release.
 
 ### Metrics
 
+- Specify how to handle instrument name conflicts.
+  ([#3606](https://github.com/open-telemetry/opentelemetry-specification/pull/3606))
+
 ### Logs
 
 ### Resource

specification/metrics/sdk.md

@@ -42,6 +42,7 @@ linkTitle: SDK
     + [Asynchronous instrument cardinality limits](#asynchronous-instrument-cardinality-limits)
 - [Meter](#meter)
   * [Duplicate instrument registration](#duplicate-instrument-registration)
+    + [Name conflict](#name-conflict)
   * [Instrument name](#instrument-name)
   * [Instrument unit](#instrument-unit)
   * [Instrument description](#instrument-description)
@@ -768,40 +769,55 @@ Distinct meters MUST be treated as separate namespaces for the purposes of detec
 
 ### Duplicate instrument registration
 
-When more than one Instrument of the same `name` is created for identical
-Meters from the same MeterProvider, denoted _duplicate instrument
-registration_, the Meter MUST create a valid Instrument in every case. Here,
-"valid" means an instrument that is functional and can be expected to export
-data, despite potentially creating a [semantic error in the data
-model](data-model.md#opentelemetry-protocol-data-model-producer-recommendations).
-
-It is unspecified whether or under which conditions the same or
-different Instrument instance will be returned as a result of
-duplicate instrument registration. The term _identical_ applied to
-Instruments describes instances where all [identifying
-fields](./api.md#instrument) are equal.  The term _distinct_ applied
-to Instruments describes instances where at least one field value is
-different.
+A _duplicate instrument registration_ occurs when more than one Instrument of
+the same [`name`](./api.md#instrument-name-syntax) is created for identical
+Meters from the same MeterProvider but they have different [identifying
+fields](./api.md#instrument).
 
-To accommodate [the recommendations from the data
-model](data-model.md#opentelemetry-protocol-data-model-producer-recommendations),
-the SDK MUST aggregate data from [identical Instruments](api.md#instrument)
-together in its export pipeline.
+Whenever this occurs, users still need to be able to make measurements with the
+duplicate instrument. This means that the Meter MUST return a functional
+instrument that can be expected to export data even if this will cause
+[semantic error in the data
+model](data-model.md#opentelemetry-protocol-data-model-producer-recommendations).
 
-When a duplicate instrument registration occurs, and it is not corrected with a
-View, a warning SHOULD be emitted. The emitted warning SHOULD include
-information for the user on how to resolve the conflict, if possible.
+Additionally, users need to be informed about this error. Therefore, when a
+duplicate instrument registration occurs, and it is not corrected with a View,
+a warning SHOULD be emitted. The emitted warning SHOULD include information for
+the user on how to resolve the conflict, if possible.
 
 1. If the potential conflict involves multiple `description`
    properties, setting the `description` through a configured View
    SHOULD avoid the warning.
-2. If the potential conflict involves instruments that can be
-   distinguished by a supported View selector (e.g., instrument type)
-   a renaming View recipe SHOULD be included in the warning.
+2. If the potential conflict involves instruments that can be distinguished by
+   a supported View selector (e.g. name, instrument kind) a renaming View
+   recipe SHOULD be included in the warning.
 3. Otherwise (e.g., use of multiple units), the SDK SHOULD pass through the
    data by reporting both `Metric` objects and emit a generic warning
    describing the duplicate instrument registration.
 
+Outside of the following [sub-section](#name-conflict), it is unspecified
+whether or under which conditions the same or different Instrument instance
+will be returned as a result of duplicate instrument registration. The term
+_identical_ applied to Instruments describes instances where all [identifying
+fields](./api.md#instrument) are equal.  The term _distinct_ applied to
+Instruments describes instances where at least one field value is different.
+
+To accommodate [the recommendations from the data
+model](data-model.md#opentelemetry-protocol-data-model-producer-recommendations),
+the SDK MUST aggregate data from [identical Instruments](api.md#instrument)
+together in its export pipeline.
+
+#### Name conflict
+
+The [`name`](./api.md#instrument-name-syntax) of an Instrument is defined to be
+case-insensitive. If an SDK uses a case-sensitive encoding to represent this
+`name`, a duplicate instrument registration will occur when a user passes
+multiple casings of the same `name`. When this happens, the Meter MUST return
+distinct instruments for each casing and log an appropriate error as described
+above. Unifying of the names is expected to be better handled "downstream" in
+an exporter or collection system that will have better context for how to
+handle the conflict.
+
 ### Instrument name
 
 When a Meter creates an instrument, it SHOULD validate the instrument name