Replace dgraph-io/badger cache storage with etcd-io/bbolt

[stefans-elastic]

Proposed commit message

Replacing dgraph-io/badger persistent storage for key-value cache with etcd-io/bbolt. Originally it was meant to just get rid of go.opencensus.io dependency which is introduced by badger (please see parent issue for more details). After it got evident that this won't erase go.opencensus.io dependency it was decided that this work still should be done since etcd-io/bbolt is already used elsewhere in the project and it isn't a good thing to have multiple storages for cache (again, please see parent issue for more details (in comments)).

Implementation should be fairly straight-forward but I would like to clarify one thing - since bolt doesn't support value expiration the expiration time (and TTL) are stored as metadata of the value. Upon value retrieval it is checked for expiration and if it is expired then nil is returned and value gets deleted from bolt DB.

Checklist

• My code follows the style guidelines of this project
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have made corresponding change to the default configuration files
• I have added tests that prove my fix is effective or that my feature works
• I have added an entry in CHANGELOG.next.asciidoc or CHANGELOG-developer.next.asciidoc.

Disruptive User Impact

Author's Checklist

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How to test this PR locally

Related issues

• Closes Drop dbadger-io dependency #42443

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[mergify]

This pull request does not have a backport label.
If this is a bug or security fix, could you label this PR @stefans-elastic? 🙏.
For such, you'll need to label your PR with:

• The upcoming major version of the Elastic Stack
• The upcoming minor version of the Elastic Stack (if you're not pushing a breaking change)

To fixup this pull request, you need to add the backport labels for the needed
branches, such as:

• backport-8./d is the label to automatically backport to the 8./d branch. /d is the digit

[botelastic]

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[elasticmachine]

Pinging @elastic/elastic-agent-data-plane (Team:Elastic-Agent-Data-Plane)

[AndersonQ]

[Suggestion]
Shouldn't the error be logged?
I see the original code also ignored the error, but I wonder if it's the best approach.
I know it's a cache and by definition it should not be a fatal error not being able to set a value in the cache, but if for some reason it keeps happening, it might impact the overall performance of whatever is using the cache. Besides if it fails often, there might be an issue that needs to be investigated.

[stefans-elastic]

i'm not sure, I wasn't intending to change the code (had to due to linter complains). Do you want me to add logging of the error?

[AndersonQ]

[NIT]
It's not connecting really connecting somewhere, perhaps Open is more descriptive as it operates on files

[stefans-elastic]

yes but the name comes from interface which is intended as abstraction (bbolt is just an implementation, another implementation maight be redis or or nats.io something different). At least this was my reasoning...