Allow multiple semconv in --only flag (#157)

* allowing multiple only flags to generate multiple conventions

* oops

* review comments

* lint

* lint

* validate --only

* lint

semantic-conventions/src/opentelemetry/semconv/main.py

@@ -51,12 +51,12 @@ def exclude_file_list(folder: str, pattern: str) -> List[str]:
     return file_names
 
 
-def filter_semconv(semconv, type_filter):
-    if type_filter:
+def filter_semconv(semconv, types):
+    if types:
         semconv.models = {
             id: model
             for id, model in semconv.models.items()
-            if model.GROUP_TYPE_NAME == type_filter
+            if model.GROUP_TYPE_NAME in types
         }
 
 
@@ -65,7 +65,8 @@ def main():
     args = parser.parse_args()
     check_args(args, parser)
     semconv = parse_semconv(args, parser)
-    filter_semconv(semconv, args.only)
+    semconv_filter = parse_only_filter(args, parser)
+    filter_semconv(semconv, semconv_filter)
     if len(semconv.models) == 0:
         parser.error("No semantic convention model found!")
     if args.flavor == "code":
@@ -109,6 +110,20 @@ def check_args(arguments, parser):
         parser.error("Either --yaml-root or YAML_FILE must be present")
 
 
+def parse_only_filter(arguments, parser):
+    if not arguments.only:
+        return None
+
+    types = [t.strip() for t in arguments.only.split(",")]
+    unknown_types = [t for t in types if t not in CONVENTION_CLS_BY_GROUP_TYPE.keys()]
+    if unknown_types:
+        parser.error(
+            f"Unknown semconv names in `--only` option: '{', '.join(unknown_types)}'"
+        )
+        sys.exit(1)
+    return types
+
+
 def add_code_parser(subparsers):
     parser = subparsers.add_parser("code")
     parser.add_argument(
@@ -210,8 +225,18 @@ def setup_parser():
     )
     parser.add_argument(
         "--only",
-        choices=list(CONVENTION_CLS_BY_GROUP_TYPE.keys()),
-        help="Process only semantic conventions of the specified type.",
+        type=str,
+        help=f"""Generates semantic conventions of the specified types only
+        ({", ".join(CONVENTION_CLS_BY_GROUP_TYPE.keys())}).
+
+        To generate multiple conventions at once, pass comma-separated list of
+        convention names, e.g. '--only span,event'.
+
+        The `--only` flag filters the output and does not apply to input.
+        Resolution of referenced attributes (using `ref`), or semantic conventions
+        (using `exclude` or `include`) is done against all semantic convention
+        files provided as input using `--yaml-root` or `YAML_FILE` options.
+        """,
     )
     parser.add_argument(
         "--yaml-root",