docs(policy): add rego examples for OTel policies

docs/proposals/policy-engine.md

@@ -33,17 +33,115 @@ declarative language. The entire set of OpenTelemetry policies could be
 expressed in a `Rego` file, versioned in the semantic conventions repository. 
 Policies specific to a company could be expressed in another versioned `Rego`
 file in the company's repository. The Weaver tool could be extended to verify
-these policies during schema resolution. The `weaver check registry` command
-could be expanded to accept one or more `Rego` files as parameters,
-representing the policies to be verified in a specific context.
+these policies in various phases (e.g. before or after resolution). The
+`weaver check registry` command could be expanded to accept one or more `Rego`
+files as parameters, representing the policies to be verified in a specific
+context.
 
 The '[regorus](https://github.com/microsoft/regorus)' project by Microsoft could be used to implement this feature
-without having a dependency on the OPA toolchain.
+without having a dependency on the OPA toolchain, making weaver easy to use in
+any CI/CD pipeline, such as OpenTelemetry or pipelines of any vendor/company.
+
+# Policies on unresolved semantic conventions
+
+The policy verification could operate as follows:
+- Read semconv files of the new version
+- Read semconv files of the previous version (if exists)
+- Apply `rego` policies on these two inputs
+- Display detected violations
+
+Example of a policy expressed in `Rego`:
+```rego
+package otel
+
+# A registry attribute groups containing at least one `ref` attribute is considered invalid.
+violations[violation] {
+    group := data.groups[_]
+    startswith(group.id, "registry.")
+    attr := group.attributes[_]
+    attr.ref != null
+    violation := {
+        "violation": "invalid_registry_ref_attribute",
+        "group": group.id,
+        "attr": attr.ref,
+        "severity": "high",
+        "category": "registry"
+    }
+}
+
+# An attribute marked as stable and deprecated is invalid.
+violations[violation] {
+    group := data.groups[_]
+    attr := group.attributes[_]
+    attr.stability == "stable"
+    attr.deprecated
+    violation := {
+        "violation": "invalid_attribute_deprecated_stable",
+        "group": group.id,
+        "attr": attr.id,
+        "severity": "high",
+        "category": "attribute"
+    }
+}
+
+# other violations rules here...
+```
+
+These policies applied to the following semconv file...
+```yaml
+groups:
+  - id: registry.network
+    prefix: network
+    type: attribute_group
+    brief: >
+      These attributes may be used for any network related operation.
+    attributes:
+      - id: protocol.name.1
+        stability: stable
+        type: string
+        brief: '[OSI application layer](https://osi-model.com/application-layer/) or non-OSI equivalent.'
+        note: The value SHOULD be normalized to lowercase.
+        examples: ['amqp', 'http', 'mqtt']
+        deprecated: true
+      - id: protocol.name.2
+        stability: stable
+        type: string
+        brief: '[OSI application layer](https://osi-model.com/application-layer/) or non-OSI equivalent.'
+        note: The value SHOULD be normalized to lowercase.
+        examples: ['amqp', 'http', 'mqtt']
+      - ref: protocol.port
+        deprecated: true
+```
+
+... will generate the following violations.
+
+```json
+[
+  {
+    "attr": "protocol.name.1",
+    "category": "attribute",
+    "group": "registry.network",
+    "severity": "high",
+    "violation": "invalid_attribute_deprecated_stable"
+  },
+  {
+    "attr": "protocol.port",
+    "category": "registry",
+    "group": "registry.network",
+    "severity": "high",
+    "violation": "invalid_registry_ref_attribute"
+  }
+]
+```
+
+`severity` and `category` fields are just an attempt to categorize the
+violations and could be removed if not needed.
+
+
+# Policies on resolved semantic conventions
 
 The policy verification could operate as follows:
-- Resolution of the new version of the semconv registry (or the app telemetry
-schema)
-- Resolution of the previous version of the semconv registry (or the app
-telemetry schema)
-- Application of policies on these two resolved schemas
-- Display of errors if the policies are not adhered to
+- Read and Resolve semconv files of the new version
+- Read and Resolve semconv files of the previous version (if exists)
+- Apply `rego` policies on these two resolved schemas
+- Display detected violations