GenevaExporter - LogSerialization Fixes

[cijothomas]

1. Fixes an issue which dropped the LogRecord.State, when it is a KVP of size1

2. Refactored LogExporter code a bit:
   LogExporter obtains the IReadOnlyList<KeyValuePair<string, object>> by either casting the State, or from the StateValues.
   Serializes each KVP to form Part C fields.
   Body is only populated if IncludeFormatMessage is true OR special casing {OriginalFormat} from Log extension methods.

As a result:
The below log statement, will result in empty body, as there is no formatter provided by the user, and Exporter will not attempt "ToString() on the state" to populate body.
logger.Log(LogLevel.Information, default, state: "somestringasdata", exception: null, formatter: null);

The below log statement will also result in empty body, if IncludeFormattedMessage = false, else uses the formatted message from user. Again, Exporter does not attempt any "ToString()" of the state.

logger.Log(LogLevel.Information, default, state: "somestringasdata", exception: null, formatter: (state, ex) => "Example formatted message.");

In short, Exporter no longer attempts to do "ToString() on state to populate body.".

3. Tests are refactored. (could be further improved, coming in next PR), categorized into "Raw Log method", vs "using extension methods on Log (with message templating)."

4. Expecting slight perf boost (less if...else ladder), will run benchmarks and update numbers, before merging.
// * Summary *

BenchmarkDotNet=v0.13.1, OS=Windows 10.0.19044.1586 (21H2)
Intel Core i7-4790 CPU 3.60GHz (Haswell), 1 CPU, 8 logical and 4 physical cores
.NET SDK=6.0.201
[Host] : .NET 6.0.3 (6.0.322.12309), X64 RyuJIT
DefaultJob : .NET 6.0.3 (6.0.322.12309), X64 RyuJIT

Main branch

Method	Mean	Error	StdDev	Gen 0	Allocated
LoggerWithGenevaExporter	1,100.1 ns	14.49 ns	12.85 ns	0.0610	256 B
SerializeLogRecord	797.9 ns	7.26 ns	6.43 ns	0.0057	24 B

with this PR

Method	Mean	Error	StdDev	Gen 0	Allocated
LoggerWithGenevaExporter	1,085.7 ns	14.71 ns	13.04 ns	0.0610	256 B
SerializeLogRecord	794.0 ns	7.82 ns	6.93 ns	0.0057	24 B

[reyang]

LGTM.

[codecov]

Codecov Report

Merging #295 (30d97bb) into main (3f965ae) will decrease coverage by 0.14%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main     #295      +/-   ##
==========================================
- Coverage   24.46%   24.32%   -0.15%     
==========================================
  Files         120      120              
  Lines        3687     3680       -7     
==========================================
- Hits          902      895       -7     
  Misses       2785     2785              

Impacted Files	Coverage Δ
...OpenTelemetry.Exporter.Geneva/GenevaLogExporter.cs	90.00% <100.00%> (-0.38%)	⬇️

[utpilla]

I think the CHANGELOG needs to be updated.