feat: streaming Value STRING only quotes in aggregates

[devjgm]

Fixes: #1391

This PR changes the way Value(string) streams its values.
Specifically, double quotes are no longer included when streaming scalar
Value(string) values. For example: std::cout << Value("foo") would
output foo (no quotes). This behavior matches that of std::cout << std::string("foo").

However, when streaming strings as part of an aggregate type
(std::vector or std::tuple), strings will be wrapped in double quotes
so that the contents of the string does not break the
structure/formatting of the printed array/struct itself. Any double
quotes in the string itself will be backslash escaped.

This PR also adds double-quoting to field names in tuples, which are
strings and should therefore also be quoted like other string fields in
aggregates.

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This change is 

[codecov]

Codecov Report

Merging #1401 into master will decrease coverage by 0.02%.
The diff coverage is 96.77%.

@@            Coverage Diff             @@
##           master    #1401      +/-   ##
==========================================
- Coverage   94.38%   94.35%   -0.03%     
==========================================
  Files         190      191       +1     
  Lines       15659    15711      +52     
==========================================
+ Hits        14779    14824      +45     
- Misses        880      887       +7

Impacted Files	Coverage Δ
google/cloud/spanner/sql_statement_test.cc	100% <100%> (ø)	⬆️
google/cloud/spanner/value_test.cc	99.33% <100%> (+0.01%)	⬆️
google/cloud/spanner/value.cc	96.76% <95%> (-0.25%)	⬇️
...loud/spanner/internal/partial_result_set_resume.cc	90.9% <0%> (-4.55%)	⬇️
google/cloud/spanner/results.h	95.83% <0%> (-4.17%)	⬇️
google/cloud/spanner/internal/polling_loop.h	91.89% <0%> (-2.71%)	⬇️
...ud/spanner/integration_tests/client_stress_test.cc	80.55% <0%> (-1.86%)	⬇️
...loud/spanner/internal/partial_result_set_source.cc	93.24% <0%> (-1.36%)	⬇️
google/cloud/spanner/samples/samples.cc	89.38% <0%> (-0.25%)	⬇️
google/cloud/spanner/internal/session.h	100% <0%> (ø)	⬆️
... and 10 more

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[coryan]

The code looks good, one suggestion for you to consider. If it does not work I am Okay with merging as-is.

Reviewed 3 of 3 files at r1, 1 of 1 files at r2.
Reviewable status: all files reviewed, 1 unresolved discussion (waiting on @devbww and @devjgm)

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google/cloud/spanner/value.cc, line 98 at r1 (raw file):

std::ostream& EscapeQuotes(std::ostream& os, std::string const& s) {
  for (auto const& c : s) {
    if (c == '"') os << "\\";

Hmm... Is the goal to make the output usable as a string literal in SQL? Then maybe using triple quotes as a delimiter would be simpler?

https://cloud.google.com/spanner/docs/lexical#string_and_bytes_literals

[devjgm]

Reviewable status: all files reviewed, 1 unresolved discussion (waiting on @coryan and @devbww)

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google/cloud/spanner/value.cc, line 98 at r1 (raw file):

Previously, coryan (Carlos O'Ryan) wrote…

Hmm... Is the goal to make the output usable as a string literal in SQL? Then maybe using triple quotes as a delimiter would be simpler?

https://cloud.google.com/spanner/docs/lexical#string_and_bytes_literals

Producing Spanner string literals is explicitly not a goal.

Even if we wrapped strings in triple quotes, I believe we'd still need to escape triple quotes that appeared in the string data, so I'm not sure if it'd be simpler.