Histograms

This skill provides guidance and workflows for logging histograms and managing their metadata in the Chromium codebase.

Overview

When adding code that logs a histogram, you must also add metadata about that histogram and any associated enums to ensure they are properly tracked and interpreted.

Guidelines

Metadata Files

• Histograms: Metadata goes in histograms.xml.
• Enums: Metadata about enums goes in enums.xml in the same directory as the corresponding histograms.xml.

Directory Structure

Metadata files are located in subdirectories of //tools/metrics/histograms/metadata/. The subdirectory name is based on the first part of the histogram name (e.g., Platform.Foo would likely be in metadata/platform/). If the correct subdirectory is unclear, ask the user.

Ownership

Histograms must have at least two owners:

1. The user who requested the change (usually the primary owner).
2. A relevant team (if possible). If you're unsure who to list as the second owner, ask the user.

Expiry Date

Set the expiry date to 3 months in the future from the date you add the histogram, unless a specific date is more appropriate.

Workflow

1. Identify the Histogram: Determine the name of the histogram being logged or updated.
2. Locate Metadata Directory: Find the appropriate subdirectory in //tools/metrics/histograms/metadata/.
3. Update histograms.xml: Add or update the <histogram> entry.
4. Update enums.xml (if applicable): If the histogram uses an enum, ensure the corresponding <enum> and its <int> values are defined in enums.xml.
5. Verify: Check //tools/metrics/histograms/README.md for additional policies.