Add ability to enable Metric Timestamp

[billpieper]

Enable ability to enable metric timestamps

Provides an option that can be enabled to add the timestamp capability to the metrics that are output. Defaults to off (false) -- which is the existing behavior.

https://prometheus.io/docs/instrumenting/exposition_formats/

metric_name [
  "{" label_name "=" `"` label_value `"` { "," label_name "=" `"` label_value `"` } [ "," ] "}"
] value [ timestamp ]

[codecov]

Codecov Report

Merging #45 (9fda387) into master (c626536) will decrease coverage by 0.15%.
The diff coverage is 0.00%.

@@            Coverage Diff             @@
##           master      #45      +/-   ##
==========================================
- Coverage   15.84%   15.68%   -0.16%     
==========================================
  Files           5        5              
  Lines         101      102       +1     
  Branches       22       22              
==========================================
  Hits           16       16              
- Misses         85       86       +1     

Impacted Files	Coverage Δ
src/HttpRequestDurationsMiddleware.cs	0.00% <0.00%> (ø)
src/HttpRequestDurationsOptions.cs	54.54% <0.00%> (-2.60%)	⬇️

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