indexer: allow indexing an event at runtime (#4466)

The PR added a new field `index` to event attribute, that will cause indexer service to index the event if set to true.

docs/app-dev/indexing-transactions.md

@@ -76,10 +76,11 @@ func (app *KVStoreApplication) DeliverTx(req types.RequestDeliverTx) types.Resul
     events := []abci.Event{
         {
             Type: "transfer",
-            Attributes: kv.Pairs{
-                kv.Pair{Key: []byte("sender"), Value: []byte("Bob")},
-                kv.Pair{Key: []byte("recipient"), Value: []byte("Alice")},
-                kv.Pair{Key: []byte("balance"), Value: []byte("100")},
+            Attributes: []abci.EventAttribute{
+                {Key: []byte("sender"), Value: []byte("Bob")},
+                {Key: []byte("recipient"), Value: []byte("Alice")},
+                {Key: []byte("balance"), Value: []byte("100")},
+                {Key: []byte("note"), Value: []byte("nothing"), Index: true},
             },
         },
     }
@@ -98,6 +99,9 @@ Note, there are a few predefined event types:
 
 Tendermint will throw a warning if you try to use any of the above keys.
 
+The index will be added if the `Index` field of attribute is set to true. In above example, querying
+using `transfer.note` will work.
+
 ## Querying Transactions
 
 You can query the transaction results by calling `/tx_search` RPC endpoint: