support users having already configured their apm-server secret-token in kubernetes

[stuartnelson3]

Currently, users have two options for injecting their APM Server's secret token into containers:

• Provide the apm-secret token either via command line flag or custom yaml file
• Independently creating the kubernetes Secret in their desired namespaces with the name apm-server-apm-token (cf. https://github.com/elastic/apm-mutating-webhook/blob/main/apm-agent-auto-attach/templates/webhook.yaml#L35-L42)

It might be nice to allow users to, instead of supplying the secret token, supply the name of the kubernetes secret containing the token, and then mounting that value found there inside the containers being auto-instrumented.

[codinted]

Hi @stuartnelson3 , is anyone working on this? If not, is it okay if I pick this up