exporter/stackdriver: allow credentials file

[sbernierlacapitale]

Is your feature request related to a problem? Please describe.
Traces are exported to Google Cloud Trace to the application's project. It is currently impossible (?) to specify another project as the target for these traces, or to use a specific service account.

For example, the traces sent to a collector hosted on GKE cannot easily be exported to Cloud Trace in another project, using another service account.

Describe the solution you'd like
Specifying a credentials file's path in the collector config would solve this issue. This file is the JWT public key specified at https://cloud.google.com/iot/docs/how-tos/credentials/jwts.

Describe alternatives you've considered
The only alternative that I know of is to clone this repo, manually add this ability, and build a new image to deploy.

Additional context
The modified code is essentially

if cfg.CredentialsPath != "" {
    copts = append(copts, option.WithCredentialsFile(cfg.CredentialsPath))
}

The collector's config file can then contain the following:

exporters:
  stackdriver:
    project: "my_project"
    credentials_path: "/path/to/credentials.json"

If this is considered as an interesting change, I can make a PR with the changes if needed.

[andrewhsu]

from the triage session at the sig mtg today, @dashpole volunteered to be assignee of this issue

[github-actions]

This issue has been inactive for 60 days. It will be closed in 60 days if there is no activity. To ping code owners by adding a component label, see Adding Labels via Comments, or if you are unsure of which component this issue relates to, please ping @open-telemetry/collector-contrib-triagers. If this issue is still relevant, please ping the code owners or leave a comment explaining why it is still relevant. Otherwise, please close it.

[dashpole]

This will be fixed by GoogleCloudPlatform/opentelemetry-operations-go#527

[github-actions]

This issue has been inactive for 60 days. It will be closed in 60 days if there is no activity. To ping code owners by adding a component label, see Adding Labels via Comments, or if you are unsure of which component this issue relates to, please ping @open-telemetry/collector-contrib-triagers. If this issue is still relevant, please ping the code owners or leave a comment explaining why it is still relevant. Otherwise, please close it.

[github-actions]

This issue has been closed as inactive because it has been stale for 120 days with no activity.

[dashpole]

This was fixed by GoogleCloudPlatform/opentelemetry-operations-go#527