stdoutmetric: Add WithWriter and WithPrettyPrint options

[dmathieu]

While stdoutmetric technically allows setting a custom writer with WithEncoder, using it means having to export the json package within an application, while stdouttrace exposes the quite simpler WithWriter method.

This PR therefore introduces WithWriter into stdoutmetric, so a writer other than os.Stdout can be passed easily.
It also introduces WithPrettyPrint, so the package provides the same options as stdouttrace does.

[codecov]

Codecov Report

Merging #4507 (2dfde9e) into main (9bbefc6) will increase coverage by 0.0%.
The diff coverage is 100.0%.

@@          Coverage Diff          @@
##            main   #4507   +/-   ##
=====================================
  Coverage   81.3%   81.3%           
=====================================
  Files        220     220           
  Lines      17679   17689   +10     
=====================================
+ Hits       14384   14395   +11     
+ Misses      2994    2993    -1     
  Partials     301     301           

Files	Coverage Δ
exporters/stdout/stdoutmetric/config.go	92.0% <100.0%> (+4.5%)	⬆️

[pellared]

I like that the PR tries to consolidate the two exporter stdout modules 👍

[pellared]

@MadVikingGod PTAL