[chore] [exporter/splunkhec] Reuse jsoniter stream/buffer

[dmitryax]

This improves performance by avoiding buffer allocation for every json encoded HEC event.

Before:

Before:

goos: darwin
goarch: arm64
pkg: github.com/open-telemetry/opentelemetry-collector-contrib/exporter/splunkhecexporter
Benchmark_pushLogData_10_10_1024
Benchmark_pushLogData_10_10_1024-10    	   35157	     35594 ns/op	   30895 B/op	     450 allocs/op
Benchmark_pushLogData_10_10_8K
Benchmark_pushLogData_10_10_8K-10      	   12478	     96064 ns/op	   87443 B/op	    1264 allocs/op
Benchmark_pushLogData_10_10_2M
Benchmark_pushLogData_10_10_2M-10      	   12481	     97948 ns/op	   90148 B/op	    1299 allocs/op
Benchmark_pushLogData_10_200_2M
Benchmark_pushLogData_10_200_2M-10     	     571	   2220018 ns/op	 1825808 B/op	   26002 allocs/op
Benchmark_pushLogData_100_200_2M
Benchmark_pushLogData_100_200_2M-10    	      70	  14679068 ns/op	14020439 B/op	  198538 allocs/op
Benchmark_pushLogData_100_200_5M
Benchmark_pushLogData_100_200_5M-10    	      55	  18776145 ns/op	18467160 B/op	  259916 allocs/op
BenchmarkConsumeLogsRejected
BenchmarkConsumeLogsRejected-10        	    1147	   1066386 ns/op	  912826 B/op	   13004 allocs/op

After:

pkg: github.com/open-telemetry/opentelemetry-collector-contrib/exporter/splunkhecexporter
Benchmark_pushLogData_10_10_1024
Benchmark_pushLogData_10_10_1024-10    	   37986	     32484 ns/op	   25432 B/op	     416 allocs/op
Benchmark_pushLogData_10_10_8K
Benchmark_pushLogData_10_10_8K-10      	   13611	     88131 ns/op	   71820 B/op	    1167 allocs/op
Benchmark_pushLogData_10_10_2M
Benchmark_pushLogData_10_10_2M-10      	   13221	     90941 ns/op	   74053 B/op	    1199 allocs/op
Benchmark_pushLogData_10_200_2M
Benchmark_pushLogData_10_200_2M-10     	     603	   1919072 ns/op	 1483452 B/op	   24000 allocs/op
Benchmark_pushLogData_100_200_2M
Benchmark_pushLogData_100_200_2M-10    	      79	  13570264 ns/op	11297322 B/op	  183258 allocs/op
Benchmark_pushLogData_100_200_5M
Benchmark_pushLogData_100_200_5M-10    	      60	  19903358 ns/op	14860723 B/op	  239911 allocs/op
BenchmarkConsumeLogsRejected
BenchmarkConsumeLogsRejected-10        	    1239	    979921 ns/op	  740291 B/op	   12002 allocs/op