Extract account reputation from /v3/user/account

[StefanLobbenmeierObjego]

This is the API docs: https://www.twilio.com/docs/sendgrid/api-reference/users-api/get-a-users-account-information

This would be really valuable, because unlike the other stats this exporter provides, the reputation is not logged historically in Sendgrid. Also it makes sense to get an altert if the reputation drops.

[StefanLobbenmeierObjego]

Had a brief look at Sendgrid.go and first thought it might be difficult to implement this, because collectByDate returns an array and takes an arbitrary date range. But turns out the method is always called with today in

sendgrid-stats-exporter/collect.go

Line 158 in 764d40e

for _, stats := range statistics[0].Stats {

and only a single statistic is actually used. So we can just add a second method in Sendgrid.go to fetch the account data and merge the results in collect.go

I will try to do a PR later.