Avoid logging 'headers' in log messages from the vault injector

[tspearconquest]

Before this PR, log messages look like:

I1109 01:48:45.143453       1 version.go:31] "version info" version="" commit="" buildDate="2023-10-17T20:02:53Z" component="vaultenv"
I1109 01:48:45.143527       1 main.go:181] "azure key vault env injector initializing"
E1109 01:48:45.143582       1 main.go:225] "failed validating config" err="environment variable ENV_INJECTOR_ARGS_SIGNATURE not provided or empty"

After this PR, log messages look like:

"version info" version="" commit="" buildDate="2023-10-16T20:02:48Z" component="webhook"
"active settings" httpPort="8080" httpPortExternal="80" tlsPort="8443" tlsPortExternal="443" mtlsPort="9443" mtlsPortExternal="9443" serveMetrics=false authType="cloudConfig" useAuthService=true dockerInspectionTimeout=20 cloudConfigPath="/etc/kubernetes/azure.json" logLevel="0" authServiceName="" mtlsPortExternal="9443" mtlsPort="9443"
"failed to build kube config" err="invalid configuration: no configuration has been provided, try setting KUBERNETES_MASTER environment variable"

This is particularly pertinent when json format logging is enabled, because these headers are prefixed before the opening { of the json data, which breaks all json log parsing.

Getting json format logging in the vault injector is a separate issue.

[waterfoul]

Might want to add some sort of indicator to the logs to show it's from the vault injector instead of the application itself. Before I was looking for the I/E prefix to distinguish them

[tspearconquest]

Thanks, that's exactly why I pinged you to take a look. I appreciate the heads up. I agree, there should be something to distinguish these from the app, so would you say we should configure an additional field in the logs which indicates the source?

[waterfoul]

That would work perfectly. Just adding source=akv2k8s-injector would be sufficient