Remove legacy metrics, they were marked as legacy for ~12 months (#2105)

* Remove legacy metrics, they were marked as legacy for ~12 months

Fixes open-telemetry/opentelemetry-collector#1082

This PR removes:
- Legacy receiver/exporter metrics:
  - otelcol/receiver/received_spans
  - otelcol/receiver/dropped_spans
  - otelcol/receiver/received_timeseries
  - otelcol/receiver/dropped_timeseries
  - otelcol/exporter/received_spans
  - otelcol/exporter/dropped_spans
  - otelcol/exporter/received_timeseries
  - otelcol/exporter/dropped_timeseries
  - otelcol/exporter/received_logs
  - otelcol/exporter/dropped_logs
- For processors remove the legacy metrics (new metrics have the same data, different names):
  - e.g. "spans_dropped" -> "processor/spans_dropped"
  - e.g. "batch_send_size_bytes" -> "processor/batch/batch_send_size_bytes"

All the new metrics were enabled when using the --new-metrics flag. The PR also removes two flags:
- "--new-metrics"
- "--legacy-metrics"
Signed-off-by: Bogdan Drutu <[email protected]>

* Update changelog

Signed-off-by: Bogdan Drutu <[email protected]>

* Fix comments from review

Signed-off-by: Bogdan Drutu <[email protected]>

kafka_receiver.go

@@ -157,7 +157,7 @@ func (c *consumerGroupHandler) ConsumeClaim(session sarama.ConsumerGroupSession,
 			zap.String("topic", message.Topic))
 		session.MarkMessage(message, "")
 
-		ctx := obsreport.ReceiverContext(session.Context(), c.name, transport, c.name)
+		ctx := obsreport.ReceiverContext(session.Context(), c.name, transport)
 		ctx = obsreport.StartTraceDataReceiveOp(ctx, c.name, transport)
 		statsTags := []tag.Mutator{tag.Insert(tagInstanceName, c.name)}
 		_ = stats.RecordWithTags(ctx, statsTags,