Enable gRPC keep-alive by default

[lrascao]

Description

Ensures intermediary proxies/gateways (AWS NLB notably) don't silently close connections after a configurable inactivity timeout expires.

Checklist

Please make sure you've completed the relevant tasks for this PR, out of the following list:

• Code compiles correctly
• Created/updated tests
• Extended the documentation

[codecov]

Codecov Report

Attention: Patch coverage is 52.00000% with 12 lines in your changes missing coverage. Please review.

Project coverage is 60.88%. Comparing base (6c59092) to head (1bc42d3).
Report is 24 commits behind head on main.

Files with missing lines	Patch %	Lines
client/client.go	52.00%	11 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #728      +/-   ##
==========================================
- Coverage   62.52%   60.88%   -1.64%     
==========================================
  Files          56       58       +2     
  Lines        4139     4372     +233     
==========================================
+ Hits         2588     2662      +74     
- Misses       1425     1579     +154     
- Partials      126      131       +5     

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[mikeee]

I'm not a fan of adding env vars endlessly to address config options as I'm partial to the functional options pattern but I do think that having both would provide the best flexibility. I propose adding options to NewClient().

I think it would be pertinent to include some documentation of this as well, at the moment I'd hazard a guess that our sparse unexported env var declarations aren't the best for surfacing this config option.

Also raising the question - do you think an incorrect value provided should cause a failure to create a new client? Instead I think defaulting to the keepalive package's 10s when provided an invalid value and logging an error would provide less friction.