Store metadata in db3 files directly

[MichaelOrlov]

Description

We have a proposal to make rosbag2 .db3 files self-contained. To achieve this goal we need to store metadata in .db3 files directly.

Related Issues

Relates:

• Allow write/record to bare file(s) instead of directory with metadata.yaml #1074
• Store DB schema versioning in .db3 files #1108
• Store message definition in bag #782

Completion Criteria

All metadata fields stored in .db3 files directly.

Implementation Notes / Suggestions

On "Rosbag2 and Tooling WG meeting" from 2022-09-23 we discussed possible implementation and came to the following design decisions and proposals:

1. We decided to store metadata as a string in yaml format in newly created schema table from Store DB schema versioning in .db3 files #1108.
   It will be relatively easily to do since we already have all necessary methods for creating and parsing metadata from string read out from yaml file.
2. We will keep writing metadata.yaml file as before and rely on metadata from it.
3. We will fallback on metadata from .db3 file directly if metadata.yaml absent or our parser failed to read or parse data from yaml file.
   The rational for this is that it's very convenient to operate with metadata stored in yaml file. i.e. It's very convenient to open yaml file in simple text editor to see what inside or edit it if necessary.

Testing Notes / Suggestions

• Will need to add unit tests to cover read out and parsing metadata from .db3 files directly
• Will need to add unit tests to make sure that old versions of bag files supported. Need to make sure that we support backward compatibility.

[ros-discourse]

This issue has been mentioned on ROS Discourse. There might be relevant details there:

https://discourse.ros.org/t/ros-2-tsc-meeting-minutes-2022-10-13/28213/1

[ros-discourse]

This issue has been mentioned on ROS Discourse. There might be relevant details there:

https://discourse.ros.org/t/ros-2-tsc-meeting-minutes-2022-11-17/28457/1

[ros-discourse]

This issue has been mentioned on ROS Discourse. There might be relevant details there:

https://discourse.ros.org/t/ros-2-tsc-meeting-minutes-2022-12-15/28840/1