Add query metrics to decision logs

[ekarlso]

It would be a nice addition to get the query logs that you can get via the http query in the decision logs instead or in addition.

[tsandall]

OPA automatically records parse, compile, and evaluation latency for all decisions and the decision log events internal to the server currently have the metrics object on them--however, we don't copy the metrics onto the event struct that's eventually serialized and uploaded.

We need to update the plugins/logs/plugin.go#EventV1 struct to include a Metrics field of type map[string]interface{} (omitempty) and populate in the plugins/logs/plugin.go#Plugin.Log function.

We should add a test case as well that verifies the metrics show up in the batched events.

[BenderScript]

It should be straightforward after #1089 is merged. I will work on it next