[v2][storage] Implement reverse adapter to translate v2 writer to v1 (jaegertracing#6555)

## Which problem is this PR solving?
- Towards jaegertracing#6065 

## Description of the changes
- This PR implements a reverse adapter (SpanWriter) that wraps a native
v2 storage writer (tracestore.Writer) and downgrades it to implement the
v1 writer (spanstore.Writer).
- This will be used by jaegertracing#6519
to downgrade a native v2 writer so that it can be used by the v1 query
service

## How was this change tested?
- Added unit tests

## Checklist
- [x] I have read
https://github.com/jaegertracing/jaeger/blob/master/CONTRIBUTING_GUIDELINES.md
- [x] I have signed all commits
- [x] I have added unit tests for the new functionality
- [x] I have run lint and test steps successfully
  - for `jaeger`: `make lint test`
  - for `jaeger-ui`: `npm run lint` and `npm run test`

---------

Signed-off-by: Mahad Zaryab <[email protected]>

Author: mahadzaryab1

storage_v2/v1adapter/spanwriter.go

@@ -0,0 +1,33 @@
+// Copyright (c) 2025 The Jaeger Authors.
+// SPDX-License-Identifier: Apache-2.0
+
+package v1adapter
+
+import (
+	"context"
+
+	jaegerTranslator "github.com/open-telemetry/opentelemetry-collector-contrib/pkg/translator/jaeger"
+
+	"github.com/jaegertracing/jaeger/model"
+	"github.com/jaegertracing/jaeger/storage/spanstore"
+	"github.com/jaegertracing/jaeger/storage_v2/tracestore"
+)
+
+var _ spanstore.Writer = (*SpanWriter)(nil)
+
+// SpanReader wraps a tracestore.Writer so that it can be downgraded to implement
+// the v1 spanstore.Writer interface.
+type SpanWriter struct {
+	traceWriter tracestore.Writer
+}
+
+func NewSpanWriter(traceWriter tracestore.Writer) *SpanWriter {
+	return &SpanWriter{
+		traceWriter: traceWriter,
+	}
+}
+
+func (sw *SpanWriter) WriteSpan(ctx context.Context, span *model.Span) error {
+	traces, _ := jaegerTranslator.ProtoToTraces([]*model.Batch{{Spans: []*model.Span{span}}})
+	return sw.traceWriter.WriteTraces(ctx, traces)
+}

storage_v2/v1adapter/spanwriter_test.go

@@ -0,0 +1,67 @@
+// Copyright (c) 2025 The Jaeger Authors.
+// SPDX-License-Identifier: Apache-2.0
+
+package v1adapter
+
+import (
+	"context"
+	"testing"
+	"time"
+
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/mock"
+	"github.com/stretchr/testify/require"
+	"go.opentelemetry.io/collector/pdata/pcommon"
+	"go.opentelemetry.io/collector/pdata/ptrace"
+
+	"github.com/jaegertracing/jaeger/model"
+	tracestoremocks "github.com/jaegertracing/jaeger/storage_v2/tracestore/mocks"
+)
+
+func TestSpanWriter_WriteSpan(t *testing.T) {
+	tests := []struct {
+		name          string
+		mockReturnErr error
+		expectedErr   error
+	}{
+		{
+			name:          "success",
+			mockReturnErr: nil,
+			expectedErr:   nil,
+		},
+		{
+			name:          "error in translator",
+			mockReturnErr: assert.AnError,
+			expectedErr:   assert.AnError,
+		},
+	}
+
+	for _, test := range tests {
+		t.Run(test.name, func(t *testing.T) {
+			mockTraceWriter := &tracestoremocks.Writer{}
+			spanWriter := NewSpanWriter(mockTraceWriter)
+
+			now := time.Now().UTC()
+			testSpan := &model.Span{
+				TraceID:   model.NewTraceID(0, 1),
+				SpanID:    model.NewSpanID(1),
+				StartTime: now,
+				Duration:  time.Second,
+			}
+
+			traces := ptrace.NewTraces()
+			resources := traces.ResourceSpans().AppendEmpty()
+			scopes := resources.ScopeSpans().AppendEmpty()
+			span := scopes.Spans().AppendEmpty()
+			span.SetTraceID(pcommon.TraceID([16]byte{0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 1}))
+			span.SetSpanID(pcommon.SpanID([8]byte{0, 0, 0, 0, 0, 0, 0, 1}))
+			span.SetStartTimestamp(pcommon.NewTimestampFromTime(now))
+			span.SetEndTimestamp(pcommon.NewTimestampFromTime(now.Add(time.Second)))
+
+			mockTraceWriter.On("WriteTraces", mock.Anything, traces).Return(test.mockReturnErr)
+
+			err := spanWriter.WriteSpan(context.Background(), testSpan)
+			require.ErrorIs(t, err, test.expectedErr)
+		})
+	}
+}