Improve attribute handling #126

Benchdiff Results

Benchmark Command: go test ./... -run ^ -bench . -count 5 -benchtime 100ms -cpu 1,2 -benchmem

HEAD sha: de289eb

Base sha: 85e5f8f

Degraded: false

pkg: github.com/elastic/apm-data/codec
goos: linux
goarch: amd64

name	old time/op (ns/op)	±	new time/op (ns/op)	±	delta	±
Encode/format=vtproto	4511	3%	4655	3%	~	(p=0.095 n=5+5)
Encode/format=vtproto-2	2289.6	0%	2574.25	0%	+12.43%	(p=0.016 n=5+4)
Encode/format=json	26212.8	1%	26363.6	1%	~	(p=0.286 n=5+5)
Encode/format=json-2	14022	1%	14009.4	2%	~	(p=0.841 n=5+5)
Decode/format=json	125793	1%	128645	4%	~	(p=0.151 n=5+5)
Decode/format=json-2	63513.6	5%	65239.2	4%	~	(p=0.310 n=5+5)
Decode/format=vtproto	19852.8	0%	20776.4	1%	+4.65%	(p=0.008 n=5+5)
Decode/format=vtproto-2	12278.8	3%	12626.6	4%	~	(p=0.095 n=5+5)

name	old bytes/op (bytes/op)	±	new bytes/op (bytes/op)	±	delta	±
Encode/format=vtproto	2775	0%	2775	0%	~	(all equal)
Encode/format=vtproto-2	2775	0%	2775	0%	~	(all equal)
Encode/format=json	6564	0%	6564	0%	~	(all equal)
Encode/format=json-2	6564	0%	6564	0%	~	(all equal)

name	old alloc/op (B/op)	±	new alloc/op (B/op)	±	delta	±
Encode/format=vtproto	3072	0%	3072	0%	~	(all equal)
Encode/format=vtproto-2	3072	0%	3072	0%	~	(all equal)
Encode/format=json	7752	0%	7752	0%	~	(all equal)
Encode/format=json-2	7753	0%	7753	0%	~	(all equal)
Decode/format=json	14520	0%	14520	0%	~	(all equal)
Decode/format=json-2	14520	0%	14520	0%	~	(all equal)
Decode/format=vtproto	12992.4	0%	12991.6	0%	~	(p=0.159 n=5+5)
Decode/format=vtproto-2	13030.5	0%	13030.4	0%	~	(p=1.000 n=4+5)

name	old allocs/op (allocs/op)	±	new allocs/op (allocs/op)	±	delta	±
Encode/format=vtproto	1	0%	1	0%	~	(all equal)
Encode/format=vtproto-2	1	0%	1	0%	~	(all equal)
Encode/format=json	30	0%	30	0%	~	(all equal)
Encode/format=json-2	30	0%	30	0%	~	(all equal)
Decode/format=json	397	0%	397	0%	~	(all equal)
Decode/format=json-2	397	0%	397	0%	~	(all equal)
Decode/format=vtproto	348	0%	348	0%	~	(all equal)
Decode/format=vtproto-2	348	0%	348	0%	~	(all equal)

pkg: github.com/elastic/apm-data/input/elasticapm/internal/decoder
goos: linux
goarch: amd64

name	old time/op (ns/op)	±	new time/op (ns/op)	±	delta	±
NDStreamDecoder	272.36	4%	274.02	3%	~	(p=0.730 n=5+5)
NDStreamDecoder-2	179.54	40%	158.62	4%	~	(p=1.000 n=5+5)

name	old alloc/op (B/op)	±	new alloc/op (B/op)	±	delta	±
NDStreamDecoder	72	0%	72	0%	~	(all equal)
NDStreamDecoder-2	72	0%	72	0%	~	(all equal)

name	old allocs/op (allocs/op)	±	new allocs/op (allocs/op)	±	delta	±
NDStreamDecoder	4	0%	4	0%	~	(all equal)
NDStreamDecoder-2	4	0%	4	0%	~	(all equal)

pkg: github.com/elastic/apm-data/model/modeljson
goos: linux
goarch: amd64

name	old time/op (ns/op)	±	new time/op (ns/op)	±	delta	±
APMEventToJSON/to-json	14448.4	6%	14068.4	4%	~	(p=0.151 n=5+5)
APMEventToJSON/to-json-2	13513	5%	12594.6	2%	-6.80%	(p=0.008 n=5+5)

name	old alloc/op (B/op)	±	new alloc/op (B/op)	±	delta	±
APMEventToJSON/to-json	2472	0%	2472	0%	~	(all equal)
APMEventToJSON/to-json-2	2472	0%	2472	0%	~	(all equal)

name	old allocs/op (allocs/op)	±	new allocs/op (allocs/op)	±	delta	±
APMEventToJSON/to-json	37	0%	37	0%	~	(all equal)
APMEventToJSON/to-json-2	37	0%	37	0%	~	(all equal)