Update README.md

We were attempting to setup the Redpanda Dashboard in Datadog and there were a number of missing metrics.  Based upon Datadog recommendations I have included their recommendations.  Also even with this update there were some metrics that did not line up against a local curl command from within a Redpanda pod.  E.g.  redpanda_kafka_request_latency_seconds_bucket, redpanda_kafka_request_latency_seconds_count, and
redpanda_kafka_request_latency_seconds_sum.  Did not retrieve any data until I added:
"redpanda_kafka_request_latency_seconds": "kafka.request_latency_seconds".

Also even though scraping the public_metrics from withing the pods, it showed values for redpanda_kafka_request_bytes_total, however in our helm chart annotations I had to add:
"redpanda_kafka_request_bytes": "kafka.request_bytes", before any data would appear in Datadog for that metric.

I did not go through all of the metrics to see if they appear in Datadog, just the ones I needed to get the Dashboard to work.  

There also seems to be some metrics that have slightly different names from what the "stock" Datadog Repanda is expecting.   E.g.  Datadog is expecting the metric name.  redpanda.kafka.request_latency_seconds, which does not exist.  As I cannot modifiy the template I made a clone and used: redpanda.kafka.request_latency_seconds.bucket instead.  

Anyway this proposed changes covers a few select items, but I think there is more work to be done here.