[NFC] Improve scope/instrument names in metrics ostream exporter

[bsarden]

Previously, we were overloading the name key in the metrics ostream exporter. This patch qualifies the name key with the appropriate prefix based on the structures they represent.

[codecov[bot]]

Codecov Report

Merging #1544 (0170a63) into main (b4ed047) will not change coverage. The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             main    #1544   +/-   ##
=======================================
  Coverage   83.93%   83.93%           
=======================================
  Files         156      156           
  Lines        4908     4908           
=======================================
  Hits         4119     4119           
  Misses        789      789           

Impacted Files	Coverage Δ
exporters/ostream/test/ostream_metric_test.cc	100.00% <ø> (ø)
exporters/ostream/src/metric_exporter.cc	92.13% <100.00%> (ø)