custom event metadata from buckconfig

[KlotzAndrew]

metadata.rs current has a number of metadata fields added to buck2 log events, these are hardcoded and some are specific to meta. This change supports adding arbitrary metadata to buck2 event uploads, for any user to configure though their .buckconfig:

• key/values are arbitrary strings
• suports $ENV_VAR subsitution
• supports $(config section.key) directive for following for .buckconfig and .buckconfig.local
• config is loaded at daemon start, so restarts required to pickup changes

Example

.buckconfig

[buck2_metadata]
key = value
env_var = $ENV_VAR

meant to be paired with something like this to upload logs: #1296

[meta-codesync]

@facebook-github-bot has imported this pull request. If you are a Meta employee, you can view this in D101692996. (Because this pull request was imported automatically, there will not be any future comments.)