Docs: add stat endpoint description

[yuriyz]

See GluuFederation/oxAuth#1512 (comment)

[moabu]

To help :

Access monthly active users endpoint:

1. Created a new client with the below details. I didn't change any settings beside whats in the Standard settings:

   OPENID CONNECT CLIENTS DETAILS

   • Name: test
   • Client ID: 9cc66d49-31f1-4a12-b1c2-c38f77417b40
   • Subject Type: pairwise
   • ClientSecret: XXXXXXXXXXX
   • Application Type: web
   • Persist Client Authorizations: true
   • Pre-Authorization: false
   • Authentication method for the Token Endpoint: client_secret_basic
   • Logout Session Required: false
   • Include Claims In Id Token: false
   • Disabled: false
   • Scopes: [openid]
   • Grant types: [client_credentials]
   • Response types: [token]

2. Curl or add to prometheus the client details with the endpoint. the path can be /oxauth/ or /jans-auth depending if Gluu or Janssen.

curl -k https://test.gluu.org/jans-auth/restv1/internal/stat?month=202106 -u "9cc66d49-31f1-4a12-b1c2-c38f77417b40:2E585aiKkE26xmZXDXJdiI9qhTvqtKfeUXKac4z6"
{"response":{"202107":{"monthly_active_users":0,"token_count_per_granttype":{}}}}

Forexample stats for May 2021 in openmetric format would look like this:

/jans-auth/restv1/internal/stat?month=202105&format=openmetric