[bearertokenauthextension] Load token lazily for gRPC AUTH to fix token refresh issue

[ms-hujia]

Description

For the gRPC client AUTH part of bearertokenauthextension, there's a token refresh issue which causes all telemetry rejected by server after a period of time. The old token is always used even if the token is refreshed in filesystem. The root cause is the token value is cached during the PerRPCCredentials() instead of retrieving the latest. This PR aims to fix it by loading the token lazily in GetRequestMetadata().

Link to tracking issue

N/A.

Testing

After I built a new docker image with this fix and tested in my environment, the token refresh issue didn't happen again after the first token expired.

Documentation

N/A.

[frzifus]

Thanks!

[frzifus]

@ms-hujia seems we miss a changelog entry.

You can run make chlog-new locally. This will create a file in .chloggen used to generate the changelogs for the next release. When filling the template you can re-use 36749 (number of your pr) as issue number.

[ms-hujia]

@ms-hujia seems we miss a changelog entry.

You can run make chlog-new locally. This will create a file in .chloggen used to generate the changelogs for the next release. When filling the template you can re-use 36749 (number of your pr) as issue number.

Hi @frzifus, I've added the changelog and rebased with main. Could you help to merge this PR?

[ms-hujia]

Hi @jpkrohling and @frzifus, thanks for your review! I'm wondering when this fix could be merged and appear in the release.