Add prototyping requirement for significant changes (#2502)

Co-authored-by: Armin Ruech <7052238+arminru@users.noreply.github.com>

.github/PULL_REQUEST_TEMPLATE.md

@@ -11,4 +11,4 @@ Note: if the PR is touching an area that is not listed in the [existing areas](h
 * [ ] [CONTRIBUTING.md](https://github.com/open-telemetry/semantic-conventions/blob/main/CONTRIBUTING.md) guidelines followed.
 * [ ] Change log entry added, according to the guidelines in [When to add a changelog entry](https://github.com/open-telemetry/semantic-conventions/blob/main/CONTRIBUTING.md#when-to-add-a-changelog-entry).
   * If your PR does not need a change log, start the PR title with `[chore]`
-
+* [ ] Links to the prototypes or existing instrumentations (when adding or changing conventions)

CONTRIBUTING.md

@@ -52,6 +52,7 @@ Agreement](https://identity.linuxfoundation.org/projects/cncf).
 When contributing to semantic conventions, it's important to understand a few
 key, but non-obvious, aspects:
 
+- In the PR description, include links to the relevant instrumentation and any applicable prototypes. Non-trivial changes to semantic conventions should be prototyped in the corresponding instrumentation(s).
 - All attributes, metrics, etc. are formally defined in YAML files under
   the `model/` directory.
 - All descriptions, normative language are defined in the `docs/` directory.

docs/general/how-to-define-semantic-conventions.md

@@ -10,6 +10,7 @@ linkTitle: How to define new semantic conventions
 
 - [Defining new conventions](#defining-new-conventions)
   - [Best practices](#best-practices)
+    - [Prototyping](#prototyping)
     - [Defining attributes](#defining-attributes)
     - [Defining spans](#defining-spans)
     - [Defining metrics](#defining-metrics)
@@ -37,6 +38,18 @@ for new areas or make substantial changes to the existing ones.
 >
 > This section contains non-normative guidance.
 
+#### Prototyping
+
+It is strongly recommended to prototype proposed conventions in one or more instrumentations and:
+
+- validate the feasibility of collecting the proposed telemetry and attributes, ensuring the information is available and can be gathered with reasonable overhead,
+
+- confirm that the proposed terminology applies across the diverse libraries and technologies covered by the conventions,
+
+- provide actionable guidance to instrumentation authors on when and how to collect attributes and record telemetry,
+
+- evaluate how the new or updated telemetry integrates with other instrumentation layers, identifying gaps, duplication, or opportunities to improve the end-user experience.
+
 #### Defining attributes
 
 Reuse existing attributes when possible. Look through [existing conventions](/docs/registry/attributes/) for similar areas,
@@ -156,7 +169,6 @@ TBD
 
 TODO:
 
-- prototyping/implementation requirements
 - migration plan
 
 ### Migration plan