fix: Preconditions only supports %s format

[noahdietz]

Preconditions only support use of the %s format specifier (docs) and gax-java had a %d format specifier. This switches it to %s.

The following error showed up in bazel builds of java_gapic_library targets on HEAD of googleapis:

external/com_google_api_gax_java/gax-grpc/src/main/java/com/google/api/gax/grpc/InstantiatingGrpcChannelProvider.java:539: error: [PreconditionsInvalidPlaceholder] Preconditions only accepts the %s placeholder in error message strings
          poolSize <= MAX_POOL_SIZE, "Pool size must be less than %d", MAX_POOL_SIZE);
                                     ^
    (see https://errorprone.info/bugpattern/PreconditionsInvalidPlaceholder)
  Did you mean 'poolSize <= MAX_POOL_SIZE, "Pool size must be less than %s", MAX_POOL_SIZE);'?

I'm not sure what changed, maybe the dependency version brought was updated transiently to the later versions that throw an exception on format specifier support in Preconditions. Regardless, this is an easy fix.

[codecov]

Codecov Report

Merging #1153 into master will not change coverage.
The diff coverage is n/a.

@@            Coverage Diff            @@
##             master    #1153   +/-   ##
=========================================
  Coverage     78.65%   78.65%           
  Complexity     1170     1170           
=========================================
  Files           204      204           
  Lines          5195     5195           
  Branches        417      417           
=========================================
  Hits           4086     4086           
  Misses          935      935           
  Partials        174      174           

Impacted Files	Coverage Δ	Complexity Δ
...api/gax/grpc/InstantiatingGrpcChannelProvider.java	77.83% <ø> (ø)	33.00 <0.00> (ø)

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[miraleung]

Alternatively, String.format("Pool size ... %d", MAX_POOL_SIZE)