fix: rendering mock values for recursive messages no longer crashes

[software-dov]

Protobuf allows recursive message types, i.e. messages whose fields
are of the same type as the message itself.

message Foo {
    Foo foo = 1; // Degenerate case
}

A real world example is bigquery.v2.data:RowFilter

These recursive types cause a problem when trying to render
mock values for unit tests because there's no inherent limit on
when to stop rendering nested values.
The solution in this commit is an artifical cap on the depth of
recursion in rendering mock values.

[codecov]

Codecov Report

Merging #587 into master will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##            master      #587   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           26        26           
  Lines         1508      1519   +11     
  Branches       308       309    +1     
=========================================
+ Hits          1508      1519   +11     

Impacted Files	Coverage Δ
gapic/schema/wrappers.py	100.00% <100.00%> (ø)

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[busunkim96]

Does it make sense to add a message like this to https://github.com/googleapis/gapic-showcase?

[software-dov]

It definitely sounds like a good idea to add a recursive showcase message type. I'll file an issue there.

[software-dov]

Showcase issue tracked by googleapis/gapic-showcase#449