Google Cloud service accounts use a JWT token to get an
oauth access token. This adds support for that.

fix: use datasource cache for backend datasources

Fix anonymous usage stats for authentication types

[metrics]enabled = false should disable the /metrics endpoint.

Just wanted to reduce the amount of files/folders in root

[skip ci]

(prometheus) add annotation option to treat series value as timestamp 

add support for interpolate $__interval and  $__interval_ms in sql datasources

but we will still keep sending metrics to graphite

closes #10638