Document patterns and suggestions for semconv code generation (#953)

.chloggen/953.yaml

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+change_type: enhancement
+
+component: other
+
+note: Document patterns and suggestions for semconv code generation.
+
+issues: [551, 953]

supplementary-guidelines/semantic_conventions_code_generation.md

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+# Semantic convention libraries
+
+<!-- toc -->
+
+- [Stability and Versioning](#stability-and-versioning)
+  - [Deprecated Conventions](#deprecated-conventions)
+- [Semantic Conventions Artifact Structure](#semantic-conventions-artifact-structure)
+- [Generating semantic conventions](#generating-semantic-conventions)
+
+<!-- tocstop -->
+
+The code for OpenTelemetry Semantic Conventions defined in this repository can be auto-generated.
+
+OpenTelemetry Language SIGs can generate Semantic Conventions code in the form that's idiomatic for
+their language and may (or may not) ship it as a stand-alone library.
+
+This document outlines common patterns and provides non-normative guidance on how to structure semantic conventions artifact
+and generate the code.
+
+## Stability and Versioning
+
+Semantic Conventions contain a mix of stability levels.
+Language SIGs that ship semantic conventions library may decide to ship a stable artifact with stable part of the Semantic Conventions, a preview artifact with all Semantic Conventions, or other combination that's idiomatic for this language and provides [SemVer 2.0](https://semver.org/) stability guarantees.
+
+Possible solutions include:
+
+- Generate all Semantic Conventions for a given version in specific folder while keeping old versions intact. It is used by [opentelemetry-go](https://github.com/open-telemetry/opentelemetry-go/tree/main/semconv/) but could be problematic if the artifact size is a concern.
+- Follow language-specific conventions to annotate experimental parts. For example, Semantic Conventions in Python puts experimental attributes in `opentelemetry.semconv._incubating` import path which is considered (following Python underscore convention) to be internal and subject to change.
+- Ship two different artifacts: one that contains stable Semantic Conventions and another one with all available conventions. For example, [semantic-conventions in Java](https://github.com/open-telemetry/semantic-conventions-java) are shipped in two artifacts: `opentelemetry-semconv` and `opentelemetry-semconv-incubating`.
+
+> Note:
+> Shipping two versions of the same artifact (stable and preview) could be problematic due to diamond-dependency problems.
+> For example, if user application depends on the `semconv v1.0.0-preview` and some library brings transitive dependency on `semconv v1.1.0` that does not contain
+> experimental conventions, the latter would be resolved leading to compilation or runtime issues in the application.
+
+Instrumentation libraries should depend on the stable (part of) semantic convention artifact or copy relevant definitions into their own code base.
+Experimental semantic conventions are intended for end-user applications.
+
+### Deprecated Conventions
+
+It's recommended to generate code for deprecated attributes, metrics, and other conventions. Use appropriate annotations to mark them as deprecated.
+Conventions have a `stability` property which provide the stability level at the deprecation time (`experimental` or `stable`) and
+the `deprecated` property that describes deprecation reason which can be used to generate documentation.
+
+- Deprecated conventions that reached stability should not be removed without major version update according to SemVer.
+- Conventions that were deprecated while being experimental should still be generated and kept in the preview (part of) semantic conventions artifact. It minimizes runtime issues
+  and breaking changes in user applications.
+
+Keep stable convention definitions inside the preview (part of) semantic conversions artifact. It prevents user code from breaking when semantic convention stabilizes. Deprecate stable definitions inside the preview artifact and point users to the stable location in generated documentation.
+For example, in Java `http.request.method` attribute is defined as the deprecated `io.opentelemetry.semconv.incubating.HttpIncubatingAttributes.HTTP_REQUEST_METHOD` field and also as stable `io.opentelemetry.semconv.HttpAttributes.HTTP_REQUEST_METHOD`.
+
+## Semantic Conventions Artifact Structure
+
+This section contains suggestions on structuring semantic convention artifact(s) which should be adjusted to the specific language.
+
+- Artifact name:
+  - `opentelemetry-semconv` - stable conventions
+  - `opentelemetry-semconv-incubating` - (if applicable) the preview artifact containing all conventions
+- Namespace: `opentelemetry.semconv` and `opentelemetry.semconv.incubating`
+- All supported Schema URLs should be listed to allow different instrumentations in the same application to provide the exact version of conventions they follow.
+- Attributes, metrics, and other convention definitions should be grouped by the convention type and the root namespace. See the example below:
+
+```
+├── SchemaUrls.code
+├── attributes
+│   ├── ClientAttributes.code
+│   ├── HttpAttributes.code
+│   └── ...
+├── metrics
+│   ├── HttpMetrics.code
+│   └── ...
+└── events
+    └── ...
+```
+
+## Generating semantic conventions
+
+> Note:
+> The tooling used for code generation may change to [opentelemetry-weaver](https://github.com/open-telemetry/weaver),
+> without any breaking changes in the generated code and with minimal changes to generation process and templates.
+
+The generation is done using [build-tools code generator](https://github.com/open-telemetry/build-tools/blob/main/semantic-conventions/README.md#code-generator).
+It's based on YAML definitions of the semantic conventions and uses [Jinja templates](https://palletsprojects.com/p/jinja/).
+
+For example, this Jinja template can be used to generate Python constant for an attribute name along with the docstring.
+
+```jinja
+{{attribute.fqn | to_const_name}} = "{{attribute.fqn}}"
+"""
+{{attribute.brief | to_doc_brief}}.
+{%- if attribute.note %}
+Note: {{attribute.note | to_doc_brief | indent}}.
+{%- endif %}
+"""
+```
+
+It generates the following code:
+
+```python
+SERVER_ADDRESS = "server.address"
+"""
+Server domain name if available without reverse DNS lookup; otherwise, IP address or Unix domain socket name.
+Note: When observed from the client side, and when communicating through an intermediary, `server.address` SHOULD represent the server address behind any intermediaries, for example proxies, if it's available.
+"""
+```
+
+Language SIGs are expected to create Jinja templates specific to their language.
+Code-generation usually involves several steps which could be semi-automated:
+
+1. Manually update the Semantic Conventions version when necessary
+2. Add the new Schema URL to the list of supported versions
+   - If it's not automated, then it can, at least, be automatically checked.
+3. Check out (or download) this version of Semantic Conventions
+4. Run code-generation script for each template
+5. Fix lint violations in the auto-generated code (if any)
+6. Send the PR with new code to the corresponding repository
+
+Here're the examples of how steps 2-5 are implemented for [Java](https://github.com/open-telemetry/semantic-conventions-java/blob/7da24068eea69dff11a78d59750b115dc4c5854d/build.gradle.kts#L55-L137) and [Python](https://github.com/open-telemetry/opentelemetry-python/blob/397e357dfad3e6ff42c09c74d5945dfdcad24bdd/scripts/semconv/generate.sh).