[crash] stdout json exporter crashes when many spans generated at once

[lizthegrey]

Description

Crash due to insufficient locking in #2196 which switched from a thread-safe JSON package method to a non-thread-safe JSON exporter object that uses a single buffer

Environment

• OS: Linux
• Architecture: amd64
• Go Version: 1.17.1
• opentelemetry-go version: v1.0.0

Steps To Reproduce

Generate many spans at once, with JSON and pretty-printing on, and simplespanexporter used as the export method e.g. https://glitch.com/edit/#!/intro-to-o11y-go?path=src%2Fmain.go%3A85%3A37

Expected behavior

No crash, all spans exported successfully and in sequence

Observed behavior

goroutine 71 [running]:
bytes.(*Buffer).WriteByte(0xc000172cb0, 0xc0)
	/tmp/go/src/bytes/buffer.go:269 +0x85
encoding/json.newline(0xc0004c4340, {0x0, 0x0}, {0xa199c0, 0x1}, 0x3)
	/tmp/go/src/encoding/json/indent.go:63 +0x45
encoding/json.Indent(0xc0005de330, {0xc00048e000, 0x439, 0x0}, {0x0, 0x0}, {0xa199c0, 0x1})
	/tmp/go/src/encoding/json/indent.go:118 +0x245
encoding/json.(*Encoder).Encode(0xc0000889b0, {0x935fe0, 0xc000142300})
	/tmp/go/src/encoding/json/stream.go:225 +0x18b
go.opentelemetry.io/otel/exporters/stdout/stdouttrace.(*Exporter).ExportSpans(0xc0003250b0, {0x98ef9b6c6f889cc8, 0xe096794a2d1d49d}, {0xc0005e1760, 0x0, 0x1})
	/app/pkg/mod/go.opentelemetry.io/otel/exporters/stdout/stdouttrace@v1.0.0/trace.go:86 +0x1dc
go.opentelemetry.io/otel/sdk/trace.(*simpleSpanProcessor).OnEnd(0xc00007d580, {0xac0df8, 0xc000142180})
	/app/pkg/mod/go.opentelemetry.io/otel/sdk@v1.0.0/trace/simple_span_processor.go:58 +0x1e6
go.opentelemetry.io/otel/sdk/trace.(*recordingSpan).End(0xc000142000, {0x0, 0x0, 0x0})
	/app/pkg/mod/go.opentelemetry.io/otel/sdk@v1.0.0/trace/span.go:280 +0x9e2
go.opentelemetry.io/contrib/instrumentation/net/http/httptrace/otelhttptrace.(*clientTracer).end(0xc0004a45a0, {0xc0005f2fc0, 0xc00014b4d8}, {0x0, 0x0}, {0xc0005a3600, 0x2, 0x2})
	/app/pkg/mod/go.opentelemetry.io/contrib/instrumentation/net/http/httptrace/otelhttptrace@v0.24.0/clienttrace.go:224 +0x46d