[Exporter.Geneva] Add support for exporting metrics to different combinations of account and namespace

[utpilla]

Users can now export individual metric streams to:

• An account of their choice by adding the dimension _microsoft_metrics_account and providing a string value for it as the account name.
• A metric namespace of their choice by adding the dimension _microsoft_metrics_namespace and providing a string value for it as the namespace name.

Note: This is not required. By default, the metrics would be sent to the account/namespace values provided in the ConnectionString. This feature allows you to override this behavior selectively by adding these custom tags only to those metrics that you would like to send to some custom account/namespace different from the default ones (coming from the ConnectionString).

Changes

• Add support for exporting metrics to different combinations of account and namespace

Please provide a brief description of the changes here.

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

• Appropriate CHANGELOG.md updated for non-trivial changes

[codecov]

Codecov Report

Merging #1111 (591cc63) into main (d99d1d3) will increase coverage by 0.01%.
The diff coverage is 81.17%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1111      +/-   ##
==========================================
+ Coverage   71.25%   71.27%   +0.01%     
==========================================
  Files         229      229              
  Lines        8493     8535      +42     
==========================================
+ Hits         6052     6083      +31     
- Misses       2441     2452      +11     

Impacted Files	Coverage Δ
...ry.Exporter.Geneva/Metrics/GenevaMetricExporter.cs	80.00% <80.48%> (-0.57%)	⬇️
...rter.Geneva/Metrics/GenevaMetricExporterOptions.cs	100.00% <100.00%> (ø)

... and 1 file with indirect coverage changes

[cijothomas]

LGTM