[DO NOT SUBMIT] Decode spanner proto fields.

[keyurva]

• Please DO NOT submit this PR.
• The intention of this PR is to only demonstrate how spanner proto fields can be decoded.
• It makes the minimal changes possible to do this.
  • e.g. it converts the proto to an existing TimeSeries object so code upstream continues to work without changes. We may want to change that in production code.
  • Also, since there is no provenance field anymore, it simply uses the provenance_url field for it. This obviously needs to change.
• The temp main file can be used to run and test this change.