chore(ci): add action to test abnf syntax and examples in OM2.0 spec

Action item from OpenMetrics 2.0 WG.

Signed-off-by: György Krajcsovits <gyorgy.krajcsovits@grafana.com>

.github/workflows/openmetrics.yml

@@ -0,0 +1,24 @@
+name: OpenMetrics
+
+on:
+  pull_request:
+    paths:
+      - 'docs/specs/om/open_metrics_spec_2_0.md'
+
+jobs:
+  check-abnf:
+    runs-on: ubuntu-latest
+
+    steps:
+    - uses: actions/checkout@11bd71901bbe5b1630ceea73d27597364c9af683  # v4.2.2
+    - name: Set up Python 3.x
+      uses: actions/setup-python@a26af69be951a213d495a4c3e4e4022e16d87065  # v5.6.0
+      with:
+        python-version: "3.12.3"
+    - name: Install dependencies
+      run: |
+        python -m pip install --upgrade pip
+        pip install abnf
+    - name: Check ABNF for OpenMetrics 2.0
+      run: |
+        python3 scripts/check_openmetrics_spec.py docs/specs/om/open_metrics_spec_2_0.md

docs/specs/om/open_metrics_spec_2_0.md

@@ -399,7 +399,7 @@ Line endings MUST be signalled with line feed (\n) and MUST NOT contain carriage
 
 An example of a complete exposition:
 
-```
+```openmetrics
 # TYPE acme_http_router_request_seconds summary
 # UNIT acme_http_router_request_seconds seconds
 # HELP acme_http_router_request_seconds Latency though all of ACME's HTTP request router.

scripts/check_openmetrics_spec.py

@@ -0,0 +1,108 @@
+#!/bin/env python3
+#
+# This script opens a markdown file containing the OpenMetrics specification,
+# extracts the ABNF grammar from it, and checks if the grammar is valid.
+# ABNF grammer must be enclosed in
+#    ```abnf
+#    exposition = metricset HASH SP eof [ LF ]
+#    ...
+#    ```
+# code block, and the top node must be `exposition`.
+# It also extracts examples from the OpenMetrics spec file and checks if they
+# are valid according to the grammar.
+# Exampes must be enclosed in
+#    ```openmetrics
+#    ... example content ...
+#    ```
+# code blocks.
+
+from abnf import Rule
+import sys
+
+class Grammar(Rule):
+    pass
+
+# Start node for the OpenMetrics spec.
+start_node = 'exposition'
+
+def get_spec(filename):
+    with open(filename, 'r') as file:
+        lines = file.readlines()
+    spec = []
+    collecting = False
+    for line in lines:
+        if collecting:
+            if line.startswith('```'):
+                collecting = False
+            else:
+                spec.append(line.strip())
+            continue
+        if line.startswith('```abnf'):
+            if len(spec) > 0:
+                raise ValueError("Multiple ABNF blocks found in the file.")
+            collecting = True
+
+    if len(spec) == 0:
+        raise ValueError("No or empty ABNF block found in the file. Wanted ```abnf ... ```.")
+    return '\n'.join(spec)
+
+
+class example:
+    def __init__(self, line_number, content):
+        self.line_number = line_number
+        self.content = content
+
+class examples:
+    """
+    Extracts examples from the OpenMetrics spec file with generator function.
+    """
+    def __init__(self, filename):
+        self.file = open(filename, 'r')
+        self.line_number = 0
+    
+    def __iter__(self):
+        return self
+
+    def __next__(self):
+        collecting = False
+        start_line = self.line_number
+        example_lines = []
+        for line in self.file:
+            self.line_number += 1
+            if collecting:
+                if line.startswith('```'):
+                    collecting = False
+                    break
+                else:
+                    example_lines.append(line)
+            elif line.startswith('```openmetrics'):
+                start_line = self.line_number
+                collecting = True
+        if len(example_lines) > 0:
+            return example(start_line, ''.join(example_lines).strip())
+
+        raise StopIteration("No more examples found.")
+
+# Main
+if __name__ == "__main__":
+    if len(sys.argv) != 2:
+        print("Usage: python3 check_openmetrics_spec.py <filename.md>")
+        sys.exit(1)
+
+    filename = sys.argv[1]
+    if not filename.endswith('.md'):
+        print(f"Error: {filename} is not a Markdown file.")
+        sys.exit(1)
+    spec = get_spec(filename)
+    try:
+        Grammar.load_grammar(grammar=spec, strict=True)
+    except Exception as e:
+        print(f"Error parsing ABNF: {e}")
+        sys.exit(1)
+    print("ABNF parsed successfully.")
+    for ex in examples(filename):
+        try:
+            Grammar.get(start_node).parse_all(ex.content)
+            print(f"Example parsed successfully: {ex.line_number}: {ex.content[:30]}...")  # Print first 30 chars
+        except Exception as e:
+            print(f"Error parsing example at line {ex.line_number}: {e}\nExample: {ex.content[:30]}...")