Fix metric SDK race condition

[jmacd]

Discovered while testing and documenting #265, this race condition between LoadOrStore and the assignment rec.recorder = i.meter.exporter.AggregatorFor(rec) could have resulted in lost updates. Fixing this would require two allocations prior to the LoadOrStore. Instead, a first call to Load checks for an existing record, followed by the LoadOrStore case. This results in fewer allocations overall, as shown in the updated benchmark.

Before:

BenchmarkAcquireNewHandle-8               	 1000000	      1192 ns/op	     275 B/op	       5 allocs/op
BenchmarkAcquireExistingHandle-8          	 2201809	       612 ns/op	     112 B/op	       2 allocs/op
BenchmarkAcquireReleaseExistingHandle-8   	 2219748	       590 ns/op	     112 B/op	       2 allocs/op

After:

BenchmarkAcquireNewHandle-8               	 1194990	      1157 ns/op	     255 B/op	       5 allocs/op
BenchmarkAcquireExistingHandle-8          	 5107620	       248 ns/op	       0 B/op	       0 allocs/op
BenchmarkAcquireReleaseExistingHandle-8   	 5356180	       253 ns/op	       0 B/op	       0 allocs/op