sdk/log: Remove slice allocation from SimpleProcessor.OnEmit

[pellared]

The reason for this improvement (apart that, in general, it is good to have better performance) is there may be good use case to use a SimpleProcessor to emit logs efficiently to standard output. It could be one of the most efficient solutions (from application performance perspective) and thanks to such configuration the user would not lose any logs if the application suddenly crashes. For instance, a useful configuration could be a simple processor with an OTLP file exporter (#5408).

I think we might consider changing the following portion of SimpleProcessor documentation (but I would prefer to do it as separate PR):

// This Processor is not recommended for production use. The synchronous
// nature of this Processor make it good for testing, debugging, or
// showing examples of other features, but it can be slow and have a high
// computation resource usage overhead. [NewBatchProcessor] is recommended
// for production use instead.

goos: linux
goarch: amd64
pkg: go.opentelemetry.io/otel/sdk/log
cpu: Intel(R) Core(TM) i9-10885H CPU @ 2.40GHz
                                    │   old.txt   │               new.txt                │
                                    │   sec/op    │    sec/op     vs base                │
Processor/Simple-16                   449.4n ± 7%   156.2n ±  5%  -65.25% (p=0.000 n=10)
Processor/ModifyTimestampSimple-16    468.0n ± 6%   171.3n ± 15%  -63.40% (p=0.000 n=10)
Processor/ModifyAttributesSimple-16   515.8n ± 3%   233.2n ±  8%  -54.77% (p=0.000 n=10)
geomean                               476.9n        184.1n        -61.40%

                                    │   old.txt   │                 new.txt                 │
                                    │    B/op     │    B/op     vs base                     │
Processor/Simple-16                    417.0 ± 0%     0.0 ± 0%  -100.00% (p=0.000 n=10)
Processor/ModifyTimestampSimple-16     417.0 ± 0%     0.0 ± 0%  -100.00% (p=0.000 n=10)
Processor/ModifyAttributesSimple-16   465.00 ± 0%   48.00 ± 0%   -89.68% (p=0.000 n=10)
geomean                                432.4                    ?                       ¹ ²
¹ summaries must be >0 to compute geomean
² ratios must be >0 to compute geomean

                                    │  old.txt   │                 new.txt                 │
                                    │ allocs/op  │ allocs/op   vs base                     │
Processor/Simple-16                   1.000 ± 0%   0.000 ± 0%  -100.00% (p=0.000 n=10)
Processor/ModifyTimestampSimple-16    1.000 ± 0%   0.000 ± 0%  -100.00% (p=0.000 n=10)
Processor/ModifyAttributesSimple-16   2.000 ± 0%   1.000 ± 0%   -50.00% (p=0.000 n=10)
geomean                               1.260                    ?                       ¹ ²
¹ summaries must be >0 to compute geomean
² ratios must be >0 to compute geomean

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 84.6%. Comparing base (6a0fa3c) to head (a428377).

Additional details and impacted files

@@           Coverage Diff           @@
##            main   #5493     +/-   ##
=======================================
- Coverage   84.6%   84.6%   -0.1%     
=======================================
  Files        268     268             
  Lines      17843   17852      +9     
=======================================
+ Hits       15103   15110      +7     
- Misses      2427    2429      +2     
  Partials     313     313             

Files	Coverage Δ
sdk/log/simple.go	100.0% <100.0%> (ø)

... and 1 file with indirect coverage changes