[pkg/ottl] Add Base64Decode Converter

[DougManton]

Component(s)

pkg/ottl

Is your feature request related to a problem? Please describe.

OTTL has no way to convert a base64 encoded string to plaintext, preventing the import of logs where the vendor encodes strings

Describe the solution you'd like

Add a new Converter, Base64Decode, that accepts a base64 encoded string and returns the decoded string.

Describe alternatives you've considered

Intermediary software to process inbound logs and decode base64 strings before passing to opentelemetry collector

Additional context

I am currently running a local fork with my implementation of a base64 decode function, I will create a branch and link it here.

[github-actions]

Pinging code owners:

• pkg/ottl: @TylerHelmuth @kentquirk @bogdandrutu @evan-bradley

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[TylerHelmuth]

Seems reasonable to me, and so would the inverse. We typically stick with specific functions, so I am ok with a specific Base64Decode instead of a generic Decode.

[evan-bradley]

Closing as resolved via #31730. If you'd like to also add an encoding function, please feel free to reopen this issue or create another issue to track it.