Allow reading a single method from the context
This adds an accessor to get a single attribute from the context to log or use in metrics.
This allows access to a single attribute without having to materialise the entire context.
We're using this for gitlab-org/gitlab!55564 (merged), in which we want to add context information to the marginalia comments. But if one of the context fields would query when being read, this would cause a circular reference.
Related to gitlab-org/gitlab#323161 (closed)
Edited by Bob Van Landuyt