Single-execution metric example app

[jiwen624]

Someone is asking for the usage of the custom metrics API. We don't seem to have a custom metrics demo app so I just added one.

Borgosz, Szymon 03:30
H! Do we have any documentation on how to use the metrics from https://github.com/appoptics/appoptics-apm-go in a single-execution Go application that does some stuff and exits to make sure those metrics are pushed to AO before the application closes? It looks like adding e.g. time.Sleep(time.Minute) before ending the program does kind-of work and agent seems to have enough time to push the metrics to AO, but is it actually the proper way of handling this use-case?

[codecov-commenter]

Codecov Report

Merging #183 (47c0da7) into master (914d2ae) will decrease coverage by 0.02%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #183      +/-   ##
==========================================
- Coverage   81.94%   81.91%   -0.03%     
==========================================
  Files          40       40              
  Lines        4292     4292              
==========================================
- Hits         3517     3516       -1     
- Misses        638      639       +1     
  Partials      137      137              

Impacted Files	Coverage Δ
v1/ao/internal/reporter/reporter_grpc.go	76.01% <0.00%> (-0.17%)	⬇️

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