Add sentry_dsn to error tracking API

Expose sentry_dsn method to error tracking client keys API.

Changelog: added
Signed-off-by: Dmytro Zaporozhets <dzaporozhets@gitlab.com>

doc/api/error_tracking.md

@@ -96,12 +96,14 @@ Example response:
   {
     "id": 1,
     "active": true,
-    "public_key": "glet_aa77551d849c083f76d0bc545ed053a3"
+    "public_key": "glet_aa77551d849c083f76d0bc545ed053a3",
+    "sentry_dsn": "https://glet_aa77551d849c083f76d0bc545ed053a3@gitlab.example.com/api/v4/error_tracking/collector/5"
   },
   {
     "id": 3,
     "active": true,
-    "public_key": "glet_0ff98b1d849c083f76d0bc545ed053a3"
+    "public_key": "glet_0ff98b1d849c083f76d0bc545ed053a3",
+    "sentry_dsn": "https://glet_0ff98b1d849c083f76d0bc545ed053a3@gitlab.example.com/api/v4/error_tracking/collector/5"
   }
 ]
 ```
@@ -129,7 +131,8 @@ Example response:
 {
   "id": 3,
   "active": true,
-  "public_key": "glet_0ff98b1d849c083f76d0bc545ed053a3"
+  "public_key": "glet_0ff98b1d849c083f76d0bc545ed053a3",
+  "sentry_dsn": "https://glet_0ff98b1d849c083f76d0bc545ed053a3@gitlab.example.com/api/v4/error_tracking/collector/5"
 }
 ```
 

lib/api/entities/error_tracking.rb

@@ -15,6 +15,7 @@ module API
         expose :id
         expose :active
         expose :public_key
+        expose :sentry_dsn
       end
     end
   end

spec/requests/api/error_tracking_client_keys_spec.rb

@@ -63,6 +63,7 @@ RSpec.describe API::ErrorTrackingClientKeys do
 
         expect(json_response['id']).to eq(new_key.id)
         expect(json_response['public_key']).to eq(new_key.public_key)
+        expect(json_response['sentry_dsn']).to eq(new_key.sentry_dsn)
       end
     end
   end