Remove convTslice calls in Record()

This is built upon
census-instrumentation#1267; that
one should likely merge first. I split this out as it has a small public
API change (to work around circular imports) to avoid issues on the
first PR.

Benchmark relative to census-instrumentation#1267:
```
me                    old time/op    new time/op    delta
Record0-6                 1.74ns ± 4%    1.79ns ± 2%   +2.85%  (p=0.238 n=5+5)
Record1-6                  634ns ± 6%     542ns ± 9%  -14.55%  (p=0.008 n=5+5)
Record8-6                 1.21µs ± 5%    1.23µs ± 2%   +1.97%  (p=0.254 n=5+5)
Record8_WithRecorder-6     777ns ± 5%     792ns ± 5%   +1.97%  (p=0.421 n=5+5)
Record8_Parallel-6        1.26µs ±24%    1.22µs ± 2%     ~     (p=0.690 n=5+5)
Record8_8Tags-6           1.23µs ± 2%    1.25µs ± 3%     ~     (p=0.651 n=5+5)

name                    old alloc/op   new alloc/op   delta
Record0-6                  0.00B          0.00B          ~     (all equal)
Record1-6                   120B ± 0%       96B ± 0%  -20.00%  (p=0.008 n=5+5)
Record8-6                   344B ± 0%      320B ± 0%   -6.98%  (p=0.008 n=5+5)
Record8_WithRecorder-6      424B ± 0%      424B ± 0%     ~     (all equal)
Record8_Parallel-6          344B ± 0%      320B ± 0%   -6.98%  (p=0.008 n=5+5)
Record8_8Tags-6             344B ± 0%      320B ± 0%   -6.98%  (p=0.008 n=5+5)

name                    old allocs/op  new allocs/op  delta
Record0-6                   0.00           0.00          ~     (all equal)
Record1-6                   3.00 ± 0%      2.00 ± 0%  -33.33%  (p=0.008 n=5+5)
Record8-6                   3.00 ± 0%      2.00 ± 0%  -33.33%  (p=0.008 n=5+5)
Record8_WithRecorder-6      4.00 ± 0%      4.00 ± 0%     ~     (all equal)
Record8_Parallel-6          3.00 ± 0%      2.00 ± 0%  -33.33%  (p=0.008 n=5+5)
Record8_8Tags-6             3.00 ± 0%      2.00 ± 0%  -33.33%  (p=0.008 n=5+5)
```

stats/record.go

@@ -86,6 +86,8 @@ func createRecordOption(ros ...Options) *recordOptions {
 	return o
 }
 
+var DefaultRecorder func(tags *tag.Map, measurement []Measurement, attachments map[string]interface{})
+
 // Record records one or multiple measurements with the same context at once.
 // If there are any tags in the context, measurements will be tagged with them.
 func Record(ctx context.Context, ms ...Measurement) {
@@ -94,7 +96,7 @@ func Record(ctx context.Context, ms ...Measurement) {
 	if len(ms) == 0 {
 		return
 	}
-	recorder := internal.DefaultRecorder
+	recorder := DefaultRecorder
 	record := false
 	for _, m := range ms {
 		if m.desc.subscribed() {

stats/view/worker.go

@@ -33,6 +33,7 @@ func init() {
 	defaultWorker = NewMeter().(*worker)
 	go defaultWorker.start()
 	internal.DefaultRecorder = record
+	stats.DefaultRecorder = defaultWorker.recordMeasurement
 }
 
 type measureRef struct {
@@ -201,9 +202,15 @@ func record(tags *tag.Map, ms interface{}, attachments map[string]interface{}) {
 
 // Record records a set of measurements ms associated with the given tags and attachments.
 func (w *worker) Record(tags *tag.Map, ms interface{}, attachments map[string]interface{}) {
+	w.recordMeasurement(tags, ms.([]stats.Measurement), attachments)
+}
+
+// recordMeasurement records a set of measurements ms associated with the given tags and attachments.
+// This is the same as Record but without an interface{} type to avoid allocations
+func (w *worker) recordMeasurement(tags *tag.Map, ms []stats.Measurement, attachments map[string]interface{}) {
 	req := &recordReq{
 		tm:          tags,
-		ms:          ms.([]stats.Measurement),
+		ms:          ms,
 		attachments: attachments,
 		t:           time.Now(),
 	}