Metrics Perf Improvement- Use KeyValuePair<string, object>[] to store dimensions

[utpilla]

On the metrics hot path, for every update call, we copy the metric dimensions received from the DiagnosticSource callback to find the right MetricPoint to be updated. This PR updates the internal structure used to store dimensions from a combination of string[] and object[] to KeyValuePair<string, object>[].

From the benchmark results, it looks like it takes lesser time to copy KeyValuePair<string, object>[] than copying both string[] and object[]. The perf benefit becomes more visible with higher number of dimensions.

Benchmark results

There is a considerable perf improvement across the board, with the perf improvement for seven dimensions (cumulative) going up to ~18%: an improvement of 60ns

BenchmarkDotNet=v0.13.3, OS=Windows 11 (10.0.22621.963)
Intel Core i7-9700 CPU 3.00GHz, 1 CPU, 8 logical and 8 physical cores
.NET SDK=7.0.101
[Host] : .NET 7.0.1 (7.0.122.56804), X64 RyuJIT AVX2
DefaultJob : .NET 7.0.1 (7.0.122.56804), X64 RyuJIT AVX2

main branch

Method	AggregationTemporality	Mean	Error	StdDev	Allocated
CounterHotPath	Cumulative	16.85 ns	0.363 ns	0.798 ns	-
CounterWith1LabelsHotPath	Cumulative	74.30 ns	0.925 ns	0.865 ns	-
CounterWith3LabelsHotPath	Cumulative	160.92 ns	3.157 ns	3.635 ns	-
CounterWith5LabelsHotPath	Cumulative	245.35 ns	4.856 ns	5.196 ns	-
CounterWith6LabelsHotPath	Cumulative	264.72 ns	1.103 ns	1.032 ns	-
CounterWith7LabelsHotPath	Cumulative	313.69 ns	4.041 ns	3.583 ns	-
CounterHotPath	Delta	15.37 ns	0.034 ns	0.030 ns	-
CounterWith1LabelsHotPath	Delta	63.10 ns	0.208 ns	0.194 ns	-
CounterWith3LabelsHotPath	Delta	149.17 ns	2.027 ns	1.896 ns	-
CounterWith5LabelsHotPath	Delta	234.79 ns	0.546 ns	0.427 ns	-
CounterWith6LabelsHotPath	Delta	261.82 ns	0.591 ns	0.524 ns	-
CounterWith7LabelsHotPath	Delta	297.78 ns	1.062 ns	0.887 ns	-

With this PR

Method	AggregationTemporality	Mean	Error	StdDev	Allocated
CounterHotPath	Cumulative	14.45 ns	0.082 ns	0.068 ns	-
CounterWith1LabelsHotPath	Cumulative	60.54 ns	0.473 ns	0.442 ns	-
CounterWith3LabelsHotPath	Cumulative	134.96 ns	1.543 ns	1.368 ns	-
CounterWith5LabelsHotPath	Cumulative	206.31 ns	1.219 ns	1.081 ns	-
CounterWith6LabelsHotPath	Cumulative	230.43 ns	0.898 ns	0.840 ns	-
CounterWith7LabelsHotPath	Cumulative	254.62 ns	1.156 ns	1.082 ns	-
CounterHotPath	Delta	13.79 ns	0.021 ns	0.019 ns	-
CounterWith1LabelsHotPath	Delta	58.06 ns	0.227 ns	0.212 ns	-
CounterWith3LabelsHotPath	Delta	131.89 ns	0.304 ns	0.285 ns	-
CounterWith5LabelsHotPath	Delta	207.92 ns	1.050 ns	0.982 ns	-
CounterWith6LabelsHotPath	Delta	232.65 ns	1.011 ns	0.896 ns	-
CounterWith7LabelsHotPath	Delta	266.57 ns	5.200 ns	5.988 ns	-

For significant contributions please make sure you have completed the following items:

• Appropriate CHANGELOG.md updated for non-trivial changes

[codecov]

Codecov Report

Merging #4059 (8480145) into main (9708349) will increase coverage by 0.05%.
The diff coverage is 81.66%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4059      +/-   ##
==========================================
+ Coverage   85.54%   85.60%   +0.05%     
==========================================
  Files         289      289              
  Lines       11259    11226      -33     
==========================================
- Hits         9632     9610      -22     
+ Misses       1627     1616      -11     

Impacted Files	Coverage Δ
src/OpenTelemetry/Metrics/ThreadStaticStorage.cs	70.83% <42.10%> (+8.86%)	⬆️
src/OpenTelemetry/Metrics/AggregatorStore.cs	82.65% <100.00%> (-0.49%)	⬇️
src/OpenTelemetry/Metrics/MetricPoint.cs	75.44% <100.00%> (ø)
src/OpenTelemetry/Metrics/Tags.cs	83.33% <100.00%> (+2.56%)	⬆️
src/OpenTelemetry/ReadOnlyTagCollection.cs	100.00% <100.00%> (ø)
...tation/OpenTelemetryProtocolExporterEventSource.cs	85.00% <0.00%> (-15.00%)	⬇️
...porter.OpenTelemetryProtocol/OtlpMetricExporter.cs	72.72% <0.00%> (-13.64%)	⬇️
...p/Implementation/HttpInstrumentationEventSource.cs	76.00% <0.00%> (+4.00%)	⬆️
...lementation/SqlClientInstrumentationEventSource.cs	75.00% <0.00%> (+4.16%)	⬆️
...Propagators/OpenTelemetryPropagatorsEventSource.cs	100.00% <0.00%> (+12.50%)	⬆️

[alanwest]

🔥