[chore] [exporter/splunkhec] Remove redundant state data and allocation

[dmitryax]

Remove redundant bufferState.index field and avoid an allocation when profiling and logs data are not send together which is the most common case

Before:

goos: darwin
goarch: arm64
pkg: github.com/open-telemetry/opentelemetry-collector-contrib/exporter/splunkhecexporter
Benchmark_pushLogData_100_10_10_1024
Benchmark_pushLogData_100_10_10_1024-10     	     410	   2702689 ns/op	 2441942 B/op	   34326 allocs/op
Benchmark_pushLogData_10_100_100_1024
Benchmark_pushLogData_10_100_100_1024-10    	     409	   2672872 ns/op	 2443447 B/op	   34506 allocs/op
Benchmark_pushLogData_10_0_100_1024
Benchmark_pushLogData_10_0_100_1024-10      	     840	   1331842 ns/op	 1226277 B/op	   17175 allocs/op
Benchmark_pushLogData_10_100_0_1024
Benchmark_pushLogData_10_100_0_1024-10      	     836	   1336186 ns/op	 1231659 B/op	   17342 allocs/op
Benchmark_pushLogData_10_10_10_256
Benchmark_pushLogData_10_10_10_256-10       	    2034	    568102 ns/op	  557723 B/op	    7689 allocs/op
Benchmark_pushLogData_10_10_10_1024
Benchmark_pushLogData_10_10_10_1024-10      	    4033	    267946 ns/op	  252992 B/op	    3456 allocs/op
Benchmark_pushLogData_10_10_10_8K
Benchmark_pushLogData_10_10_10_8K-10        	    1746	    687386 ns/op	 1861394 B/op	    2683 allocs/op
Benchmark_pushLogData_10_10_10_1M
Benchmark_pushLogData_10_10_10_1M-10        	    1555	    988593 ns/op	 6039903 B/op	    2684 allocs/op
Benchmark_pushLogData_10_1_1_1024
Benchmark_pushLogData_10_1_1_1024-10        	   41257	     27364 ns/op	   36548 B/op	     351 allocs/op
BenchmarkPushLogRecords
BenchmarkPushLogRecords-10                  	  205251	      5427 ns/op	   31409 B/op	      13 allocs/op
PASS

After:

Benchmark_pushLogData_100_10_10_1024
Benchmark_pushLogData_100_10_10_1024-10     	     446	   2550308 ns/op	 2436061 B/op	   33992 allocs/op
Benchmark_pushLogData_10_100_100_1024
Benchmark_pushLogData_10_100_100_1024-10    	     447	   2605696 ns/op	 2437463 B/op	   34172 allocs/op
Benchmark_pushLogData_10_0_100_1024
Benchmark_pushLogData_10_0_100_1024-10      	     852	   1307304 ns/op	 1222155 B/op	   17008 allocs/op
Benchmark_pushLogData_10_100_0_1024
Benchmark_pushLogData_10_100_0_1024-10      	     836	   1300739 ns/op	 1227657 B/op	   17175 allocs/op
Benchmark_pushLogData_10_10_10_256
Benchmark_pushLogData_10_10_10_256-10       	    1970	    530659 ns/op	  553186 B/op	    7489 allocs/op
Benchmark_pushLogData_10_10_10_1024
Benchmark_pushLogData_10_10_10_1024-10      	    4017	    258693 ns/op	  252180 B/op	    3422 allocs/op
Benchmark_pushLogData_10_10_10_8K
Benchmark_pushLogData_10_10_10_8K-10        	    1958	    666062 ns/op	 1861313 B/op	    2681 allocs/op
Benchmark_pushLogData_10_10_10_1M
Benchmark_pushLogData_10_10_10_1M-10        	    1567	    936454 ns/op	 6039842 B/op	    2682 allocs/op
Benchmark_pushLogData_10_1_1_1024
Benchmark_pushLogData_10_1_1_1024-10        	   41476	     27076 ns/op	   36451 B/op	     347 allocs/op
BenchmarkPushLogRecords
BenchmarkPushLogRecords-10                  	  213703	      5317 ns/op	   31408 B/op	      13 allocs/op
PASS