fix: only fail if session pool is full

[olavloite]

Only fail session creation if the pool is already full.

Fixes #751

See also:

nodejs-spanner/test/spanner.ts

Line 192 in e7edcb7

it('should reuse sessions when fail=true', async () => {

and

nodejs-spanner/src/session-pool.ts

Line 724 in e7edcb7

if (this.isFull && this.options.fail!) {

in PR #743

[codecov]

Codecov Report

Merging #752 into master will not change coverage.
The diff coverage is 100%.

@@           Coverage Diff           @@
##           master     #752   +/-   ##
=======================================
  Coverage   96.11%   96.11%           
=======================================
  Files          18       18           
  Lines       15622    15622           
  Branches      532      532           
=======================================
  Hits        15015    15015           
  Misses        602      602           
  Partials        5        5

Impacted Files	Coverage Δ
src/session-pool.ts	99.2% <100%> (ø)	⬆️

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[hengfengli]

LGTM.

[olavloite]

Closing in favor of #759 that also included this fix.