feat: a small script to plot benchmark results

[coryan]

Generate pretty (maybe) plots from the benchmark data, for example:

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This change is 

[codecov]

Codecov Report

Merging #1189 into master will increase coverage by 0.08%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #1189      +/-   ##
==========================================
+ Coverage   95.63%   95.72%   +0.08%     
==========================================
  Files         174      174              
  Lines       13567    13601      +34     
==========================================
+ Hits        12975    13019      +44     
+ Misses        592      582      -10

Impacted Files	Coverage Δ
google/cloud/spanner/internal/spanner_stub.cc	69.23% <0%> (-1.1%)	⬇️
google/cloud/spanner/client.cc	96.55% <0%> (-0.87%)	⬇️
.../spanner/benchmarks/multiple_rows_cpu_benchmark.cc	92.36% <0%> (-0.18%)	⬇️
...on_tests/rpc_failure_threshold_integration_test.cc	87.64% <0%> (-0.14%)	⬇️
...cloud/spanner/benchmarks/benchmarks_config_test.cc	100% <0%> (ø)	⬆️
google/cloud/spanner/timestamp_test.cc	100% <0%> (ø)	⬆️
google/cloud/spanner/database_admin_connection.h
google/cloud/spanner/instance_admin_connection.h	100% <0%> (ø)
...anner/integration_tests/client_integration_test.cc	98.47% <0%> (ø)	⬆️
google/cloud/spanner/value.h	92.8% <0%> (+0.1%)	⬆️
... and 3 more

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[devbww]

Nit: "storage_throughput_vs_cpu_benchmark" looks like a google-cloud-cpp remnant. Similarly in the file comment above.

[coryan]

Fixed in both places.

[coryan]

PTAL

[devjgm]

Reviewed 1 of 1 files at r2.
Reviewable status: all files reviewed, 1 unresolved discussion (waiting on @devbww, @mr-salty, and @scotthart)