loadbalancingexporter: use pdata MoveAndAppendTo()

no need to reimplement that in an extremely allocation-inefficient
fashion.

I'm actually not sure why mergeTraces() and mergeMetrics() need to exist
in the first place; all the other exporters coupled with the batch
processor work just fine, not sure why loadbalancing would be special.
#30141
seems to imply they were implemented to improve performance, but I don't
really understand why batch processor would not have been sufficient for
that improvement originally.

benchmarks before:
	goos: darwin
	goarch: arm64
	pkg: github.com/open-telemetry/opentelemetry-collector-contrib/exporter/loadbalancingexporter
	BenchmarkMergeTraces_X100-8     	   50214	     23507 ns/op
	BenchmarkMergeTraces_X500-8     	   10000	    113952 ns/op
	BenchmarkMergeTraces_X1000-8    	    5208	    226062 ns/op
	BenchmarkMergeMetrics_X100-8    	   64933	     18540 ns/op
	BenchmarkMergeMetrics_X500-8    	   12885	     91418 ns/op
	BenchmarkMergeMetrics_X1000-8   	    6590	    184584 ns/op
	PASS
	ok  	github.com/open-telemetry/opentelemetry-collector-contrib/exporter/loadbalancingexporter	9.783s

and after:
	goos: darwin
	goarch: arm64
	pkg: github.com/open-telemetry/opentelemetry-collector-contrib/exporter/loadbalancingexporter
	BenchmarkMergeTraces_X100-8     	295886529	         3.836 ns/op
	BenchmarkMergeTraces_X500-8     	309865370	         3.833 ns/op
	BenchmarkMergeTraces_X1000-8    	310739948	         3.800 ns/op
	BenchmarkMergeMetrics_X100-8    	315567813	         3.841 ns/op
	BenchmarkMergeMetrics_X500-8    	310341650	         3.849 ns/op
	BenchmarkMergeMetrics_X1000-8   	314292003	         3.830 ns/op
	PASS
	ok  	github.com/open-telemetry/opentelemetry-collector-contrib/exporter/loadbalancingexporter	10.733s

exporter/loadbalancingexporter/helpers.go

@@ -10,114 +10,12 @@ import (
 
 // mergeTraces concatenates two ptrace.Traces into a single ptrace.Traces.
 func mergeTraces(t1 ptrace.Traces, t2 ptrace.Traces) ptrace.Traces {
-	mergedTraces := ptrace.NewTraces()
-
-	if t1.SpanCount() == 0 && t2.SpanCount() == 0 {
-		return mergedTraces
-	}
-
-	// Iterate over the first trace and append spans to the merged traces
-	for i := 0; i < t1.ResourceSpans().Len(); i++ {
-		rs := t1.ResourceSpans().At(i)
-		newRS := mergedTraces.ResourceSpans().AppendEmpty()
-
-		rs.Resource().MoveTo(newRS.Resource())
-		newRS.SetSchemaUrl(rs.SchemaUrl())
-
-		for j := 0; j < rs.ScopeSpans().Len(); j++ {
-			ils := rs.ScopeSpans().At(j)
-
-			newILS := newRS.ScopeSpans().AppendEmpty()
-			ils.Scope().MoveTo(newILS.Scope())
-			newILS.SetSchemaUrl(ils.SchemaUrl())
-
-			for k := 0; k < ils.Spans().Len(); k++ {
-				span := ils.Spans().At(k)
-				newSpan := newILS.Spans().AppendEmpty()
-				span.MoveTo(newSpan)
-			}
-		}
-	}
-
-	// Iterate over the second trace and append spans to the merged traces
-	for i := 0; i < t2.ResourceSpans().Len(); i++ {
-		rs := t2.ResourceSpans().At(i)
-		newRS := mergedTraces.ResourceSpans().AppendEmpty()
-
-		rs.Resource().MoveTo(newRS.Resource())
-		newRS.SetSchemaUrl(rs.SchemaUrl())
-
-		for j := 0; j < rs.ScopeSpans().Len(); j++ {
-			ils := rs.ScopeSpans().At(j)
-
-			newILS := newRS.ScopeSpans().AppendEmpty()
-			ils.Scope().MoveTo(newILS.Scope())
-			newILS.SetSchemaUrl(ils.SchemaUrl())
-
-			for k := 0; k < ils.Spans().Len(); k++ {
-				span := ils.Spans().At(k)
-				newSpan := newILS.Spans().AppendEmpty()
-				span.MoveTo(newSpan)
-			}
-		}
-	}
-
-	return mergedTraces
+	t2.ResourceSpans().MoveAndAppendTo(t1.ResourceSpans())
+	return t1
 }
 
 // mergeMetrics concatenates two pmetric.Metrics into a single pmetric.Metrics.
 func mergeMetrics(m1 pmetric.Metrics, m2 pmetric.Metrics) pmetric.Metrics {
-	mergedMetrics := pmetric.NewMetrics()
-
-	if m1.MetricCount() == 0 && m2.MetricCount() == 0 {
-		return mergedMetrics
-	}
-
-	// Iterate over the first metric and append metrics to the merged metrics
-	for i := 0; i < m1.ResourceMetrics().Len(); i++ {
-		rs := m1.ResourceMetrics().At(i)
-		newRS := mergedMetrics.ResourceMetrics().AppendEmpty()
-
-		rs.Resource().MoveTo(newRS.Resource())
-		newRS.SetSchemaUrl(rs.SchemaUrl())
-
-		for j := 0; j < rs.ScopeMetrics().Len(); j++ {
-			ils := rs.ScopeMetrics().At(j)
-
-			newILS := newRS.ScopeMetrics().AppendEmpty()
-			ils.Scope().MoveTo(newILS.Scope())
-			newILS.SetSchemaUrl(ils.SchemaUrl())
-
-			for k := 0; k < ils.Metrics().Len(); k++ {
-				metric := ils.Metrics().At(k)
-				newMetric := newILS.Metrics().AppendEmpty()
-				metric.MoveTo(newMetric)
-			}
-		}
-	}
-
-	// Iterate over the second metric and append metrics to the merged metrics
-	for i := 0; i < m2.ResourceMetrics().Len(); i++ {
-		rs := m2.ResourceMetrics().At(i)
-		newRS := mergedMetrics.ResourceMetrics().AppendEmpty()
-
-		rs.Resource().MoveTo(newRS.Resource())
-		newRS.SetSchemaUrl(rs.SchemaUrl())
-
-		for j := 0; j < rs.ScopeMetrics().Len(); j++ {
-			ils := rs.ScopeMetrics().At(j)
-
-			newILS := newRS.ScopeMetrics().AppendEmpty()
-			ils.Scope().MoveTo(newILS.Scope())
-			newILS.SetSchemaUrl(ils.SchemaUrl())
-
-			for k := 0; k < ils.Metrics().Len(); k++ {
-				metric := ils.Metrics().At(k)
-				newMetric := newILS.Metrics().AppendEmpty()
-				metric.MoveTo(newMetric)
-			}
-		}
-	}
-
-	return mergedMetrics
+	m2.ResourceMetrics().MoveAndAppendTo(m1.ResourceMetrics())
+	return m1
 }