Improve usability of Google Cloud Operations environment variables

[NimJay]

Describe the bug

• See instructions, Integrating Online Boutique with Google Cloud.
• A Googler mentioned that they found these environment variables confusing (and thought something was broken):
  • DISABLE_STATS
  • DISABLE_TRACING
  • DISABLE_PROFILER
• The variables might be confusing because:
  • We're using double negatives. Ideally, the variable names would start with ENABLE_....
  • There is no difference between a value of 0 and a value of 1.

Additional context

• The Googler reported this issue to @Shabirmean.

Exposure

• This primarily concerns anyone that wants to enable Google Cloud Operations.
• This likely only affects a subset of GKE instances (since Google Cloud Operations is disabled by default).

[Shabirmean]

Potential improvement to the fix, link to the guide in comments.

[minherz]

@Shabirmean the provided link is not working. The file gcp-instrumentation.md is missing in the main branch.

[NimJay]

Update: We might be able to address this issue when we bring in OpenTelemtry support. CC: @arbrown

[NimJay]

Thank you, @arbrown, for the progress you've made on this so far! :)

Just leaving a screenshot of what's left over as an update: