Resolves Open Feedback re: Prohibiting Complex Types (open-telemetry#3926)

## Changes

Resolves open comments from PR (open-telemetry#3858)

1. Clarify the "why" w/o redefining metadata (See
[comment](open-telemetry#3858 (comment)))

2. Keep all the new standard attribute content together in one place

This addresses issue (open-telemetry#3925).

Author: mattmccleary

specification/common/README.md

@@ -34,7 +34,7 @@ An `Attribute` is a key-value pair, which MUST have the following properties:
 
 - The attribute key MUST be a non-`null` and non-empty string.
   - Case sensitivity of keys is preserved. Keys that differ in casing are treated as distinct keys.
-- The attribute value is either[1]:
+- The attribute value is either:
   - A primitive type: string, boolean, double precision floating point (IEEE 754-1985) or signed 64 bit integer.
   - An array of primitive type values. The array MUST be homogeneous,
     i.e., it MUST NOT contain values of different types.
@@ -66,23 +66,6 @@ See [Requirement Level](https://github.com/open-telemetry/semantic-conventions/b
 See [this document](attribute-type-mapping.md) to find out how to map values obtained
 outside OpenTelemetry into OpenTelemetry attribute values.
 
-**[1]**: NOTE: extending the set of attribute value types is a breaking change.
-This was decided after extensive debate, with arguments as follows:
-
-* Limiting the types of attribute values to a set which has proved sufficient
-  during several years of OpenTelemetry's development is a useful guardrail for
-  design. In taking additional value types off the table, we narrow the solution
-  space and have more productive design conversations.
-* We proposed extending support for complex value types and received significant
-  pushback. Removing the bounds significantly increases the burden on data
-  consumers. Adding additional simple value types doesn't cause the same level
-  of burden, but these can be encoded using existing primitive types. For
-  example, datetime can be encoded as a string or 64 bit integer.
-* Limiting attribute value types to primitives and arrays of primitives supports
-  OpenTelemetry's intent that attributes are metadata, and facilitates the
-  ability for data consumers to create search indexes and perform other
-  statistical analysis.
-
 ### Standard Attribute
 
 Attributes are used in various places throughout the OpenTelemetry data model.
@@ -98,6 +81,21 @@ reflects that LogRecord attributes are expected to model data produced from
 external log APIs, which do not necessarily have the same value type
 restrictions as the standard attribute definition.
 
+Note: Extending the set of standard attribute value types is a breaking change.
+This was decided after extensive debate, with arguments as follows:
+
+* Limiting the types of attribute values to a set which has proved sufficient
+  during several years of OpenTelemetry's development is a useful guardrail for
+  design. In taking additional value types off the table, we narrow the solution
+  space and have more productive design conversations.
+* Upon proposing to extend support for complex value types, we received significant
+  pushback. Limiting attribute value types to primitives and arrays of primitives
+  simplifies data consumers' efforts to create search indexes and perform statistical
+  analysis.
+* To address concerns over restricting standard attributes to primitive types, it was
+  called out that complex types can be encoded as existing primitive types, such as
+  representing datetime as a string or 64 bit integer.
+
 ### Attribute Limits
 
 Execution of erroneous code can result in unintended attributes. If there are no