Sub-user not used & remove double quotes from .env

[HamzaChetoui]

Hello,

1. No metrics on /metrics endpoint:
   If you call /metrics and receive no data and no logs, it likely means your API key does not have the required permissions.
   If your key does have permissions and you still see nothing:
   ==> Make sure your environment variables (SENDGRID_API_KEY, ..) do not include double quotes.
   (For example use SENDGRID_API_KEY=SG.XXX..., not SENDGRID_API_KEY="SG.XXX...")

2. Subuser metrics are missing:
   By default, SendGrid returns stats for the parent (root) account.
   To fetch metrics for a subuser, you must add this line in sendgrid.go:
   req.Header.Set("On-Behalf-Of", *sendGridUserName)