Use normative key words for Asynchronous and Synchronous API parameters

specification/metrics/api.md

@@ -322,12 +322,39 @@ pattern](https://en.wikipedia.org/wiki/Asynchronous_method_invocation).
 
 The API to construct synchronous instruments MUST accept the following parameters:
 
-* The `name` of the Instrument, following the [instrument naming
-  rule](#instrument-naming-rule).
-* An optional `unit` of measure, following the [instrument unit
-  rule](#instrument-unit).
-* An optional `description`, following the [instrument description
-  rule](#instrument-description).
+* A `name` of the Instrument.
+
+  The `name` needs to be provided by a user. If possible, the API SHOULD be
+  structured so a user is obligated to provide this parameter. If it is not
+  possible to structurally enforce this obligation, the API MUST be documented
+  in a way to communicate to users that this parameter is needed.
+
+  The `name` needs to following the [instrument naming
+  rule](#instrument-naming-rule). The API SHOULD be documented in a way to
+  communicate to users that this parameter needs to conform to the linked
+  syntax. The API MUST NOT validate the `name`, that is left to implementations
+  of the API.
+* A `unit` of measure.
+
+  Users can provide a `unit`, but it is up to their discretion. Therefore, this
+  API needs to be structured to accept a `unit`, but MUST NOT obligate a user
+  to provide one.
+
+  The `unit` parameter needs to support the [instrument unit
+  rule](#instrument-unit). Meaning, the API MUST accept a case-sensitive string
+  that supports ASCII character encoding and can hold at least 63 characters.
+  The API MUST NOT validate the `unit`.
+* A `description` describing the Instrument in human-readable terms.
+
+  Users can provide a `description`, but it is up to their discretion.
+  Therefore, this API needs to be structured to accept a `description`, but
+  MUST NOT obligate a user to provide one.
+
+  The `description` needs to support the [instrument description
+  rule](#instrument-description). Meaning, the API MUST accept a string that
+  supports at least [BMP (Unicode Plane
+  0)](https://en.wikipedia.org/wiki/Plane_(Unicode)#Basic_Multilingual_Plane)
+  encoded characters and hold at least 1023 characters.
 
 ##### Asynchronous Instrument API
 
@@ -338,14 +365,45 @@ order of callback execution is not specified.
 
 The API to construct asynchronous instruments MUST accept the following parameters:
 
-* The `name` of the Instrument, following the [instrument naming
-  rule](#instrument-naming-rule).
-* An optional `unit` of measure, following the [instrument unit
-  rule](#instrument-unit).
-* An optional `description`, following the [instrument description
-  rule](#instrument-description).
-* Zero or more `callback` functions, responsible for reporting
-  [Measurement](#measurement) values of the created instrument.
+* A `name` of the Instrument.
+
+  The `name` needs to be provided by a user. If possible, the API SHOULD be
+  structured so a user is obligated to provide this parameter. If it is not
+  possible to structurally enforce this obligation, the API MUST be documented
+  in a way to communicate to users that this parameter is needed.
+
+  The `name` needs to following the [instrument naming
+  rule](#instrument-naming-rule). The API SHOULD be documented in a way to
+  communicate to users that this parameter needs to conform to the linked
+  syntax. The API MUST NOT validate the `name`, that is left to implementations
+  of the API.
+* A `unit` of measure.
+
+  Users can provide a `unit`, but it is up to their discretion. Therefore, this
+  API needs to be structured to accept a `unit`, but MUST NOT obligate a user
+  to provide one.
+
+  The `unit` parameter needs to support the [instrument unit
+  rule](#instrument-unit). Meaning, the API MUST accept a case-sensitive string
+  that supports ASCII character encoding and can hold at least 63 characters.
+  The API MUST NOT validate the `unit`.
+* A `description` describing the Instrument in human-readable terms.
+
+  Users can provide a `description`, but it is up to their discretion.
+  Therefore, this API needs to be structured to accept a `description`, but
+  MUST NOT obligate a user to provide one.
+
+  The `description` needs to support the [instrument description
+  rule](#instrument-description). Meaning, the API MUST accept a string that
+  supports at least [BMP (Unicode Plane
+  0)](https://en.wikipedia.org/wiki/Plane_(Unicode)#Basic_Multilingual_Plane)
+  encoded characters and hold at least 1023 characters.
+* `callback` functions that report [Measurements](#measurement) of the created
+  instrument.
+
+  Users can provide `callback` functions, but it is up to their discretion.
+  Therefore, this API MUST be structured to accept a variable number of
+  `callback` functions, including none.
 
 The API MUST support creation of asynchronous instruments by passing
 zero or more `callback` functions to be permanently registered to the