out_azure_blob: add `log_key` option

[sarkisjad]

Is your feature request related to a problem? Please describe.
This feature is implemented in this PR: #9791
However, the implementation has bugs in it. The output only transports 1 line per file. It will ignore all other lines in the file.

Describe the solution you'd like
As a solution, could we fix the code in the PR

[Vaibhav-C-S]

Hi, I would like to work on this. If maintainers agree, I can raise a separate PR to add log_key support for out_azure_blob that avoids the issue where only one line is transported per file.