Limit HTTP request method cardinality: original_method (option A)

CHANGELOG.md

@@ -89,3 +89,6 @@ release.
   ([#109](https://github.com/open-telemetry/semantic-conventions/pull/109))
 - Updated AWS Java Lambda guidance - using system properties.
   ([#27](https://github.com/open-telemetry/semantic-conventions/pull/27))
+- Limit `http.request.method` values to a closed set of known values,
+  introduce `http.request.method_original` for the original value.
+  ([#17](https://github.com/open-telemetry/opentelemetry-specification/pull/17))

semantic_conventions/http-common.yaml

@@ -5,10 +5,58 @@ groups:
     prefix: http
     attributes:
       - id: request.method
-        type: string
+        type:
+          allow_custom_values: true
+          members:
+            - id: connect
+              value: "CONNECT"
+              brief: 'CONNECT method.'
+            - id: delete
+              value: "DELETE"
+              brief: 'DELETE method.'
+            - id: get
+              value: "GET"
+              brief: 'GET method.'
+            - id: head
+              value: "HEAD"
+              brief: 'HEAD method.'
+            - id: options
+              value: "OPTIONS"
+              brief: 'OPTIONS method.'
+            - id: patch
+              value: "PATCH"
+              brief: 'PATCH method.'
+            - id: post
+              value: "POST"
+              brief: 'POST method.'
+            - id: put
+              value: "PUT"
+              brief: 'PUT method.'
+            - id: trace
+              value: "TRACE"
+              brief: 'TRACE method.'
+            - id: other
+              value: "_OTHER"
+              brief: 'Any HTTP method that the instrumentation has no prior knowledge of.'
         requirement_level: required
         brief: 'HTTP request method.'
         examples: ["GET", "POST", "HEAD"]
+        note: |
+          HTTP request method value SHOULD be "known" to the instrumentation.
+          By default, this convention defines "known" methods as the ones listed in [RFC9110](https://www.rfc-editor.org/rfc/rfc9110.html#name-methods)
+          and the PATCH method defined in [RFC5789](https://www.rfc-editor.org/rfc/rfc5789.html).
+
+          If the HTTP request method is not known to instrumentation, it MUST set the `http.request.method` attribute to `_OTHER` and, except if reporting a metric, MUST
+          set the exact method received in the request line as value of the `http.request.method_original` attribute.
+
+          If the HTTP instrumentation could end up converting valid HTTP request methods to `_OTHER`, then it MUST provide a way to override
+          the list of known HTTP methods. If this override is done via environment variable, then the environment variable MUST be named
+          OTEL_INSTRUMENTATION_HTTP_KNOWN_METHODS and support a comma-separated list of case-sensitive known HTTP methods
+          (this list MUST be a full override of the default known method, it is not a list of known methods in addition to the defaults).
+
+          HTTP method names are case-sensitive and `http.request.method` attribute value MUST match a known HTTP method name exactly.
+          Instrumentations for specific web frameworks that consider HTTP methods to be case insensitive, SHOULD populate a canonical equivalent.
+          Tracing instrumentations that do so, MUST also set `http.request.method_original` to the original value.
       - id: response.status_code
         type: int
         requirement_level:
@@ -25,7 +73,7 @@ groups:
   - id: attributes.http.client
     prefix: http
     type: attribute_group
-    brief: 'HTTP client attributes'
+    brief: 'HTTP Client attributes'
     attributes:
       - ref: server.address
         requirement_level: required
@@ -51,7 +99,7 @@ groups:
   - id: attributes.http.server
     prefix: http
     type: attribute_group
-    brief: 'HTTP server attributes'
+    brief: 'HTTP Server attributes'
     attributes:
       - id: route
         type: string

semantic_conventions/trace/http.yaml

@@ -8,6 +8,12 @@ groups:
         These conventions can be used for http and https schemes
         and various HTTP versions like 1.1, 2 and SPDY.
     attributes:
+      - id: request.method_original
+        type: string
+        requirement_level:
+          conditionally_required: If and only if it's different than `http.request.method`.
+        brief: Original HTTP method sent by the client in the request line.
+        examples: ["GeT", "ACL", "foo"]
       - id: request.body.size
         type: int
         brief: >