datastore: Support excludeFromIndexes for primitives in arrays.

[beaulac]

Resolves #2615 .

This slightly modifies the index exclusion syntax discussed in #1916, which was merged in #2497.

Primitives in an array can now be excluded from indexes like this:

{
  key: key,
  data: {
      array: [
          'a string', // Excluded
          {name: 'string in entity'} // Not excluded
      ]
  },
  excludeFromIndexes: ['array[]']
}

Previously, excludeFromIndexes: ['array[]'] would have thrown a server-side error because it was interpreted as excluding the arrayValue itself from indexes, which is disallowed by the Datastore API.

This PR changes this behaviour: now, an excludeFromIndexes path ending with [] means that non-entity values in the array should be excluded.

To exclude properties of entities in arrays, the existing nested-property syntax (e.g. array[].name) is still used.

For example, to exclude both from indexes:

{
  key: key,
  data: {
      array: [
          'a string', // Excluded
          {name: 'string in entity'} // 'name' property is also excluded
      ]
  },
  excludeFromIndexes: ['array[]', 'array[].name']
}

Nested arrays remain unsupported. The following will still break, as it currently does:

{
  key: key,
  data: {
      array: [
          [1] // 'excludeFromEntities' will be set on the arrayValue, which breaks. 
      ]
  },
  excludeFromIndexes: ['array[]']
}

I wanted to change as little as possible of how entity properties are excluded, since there is no clear conclusion on how #2510 ("catch-all" index-exclusion) will be addressed. As such, I didn't want to redo the core implementation, but I realize that excludePathFromEntity is becoming rather convoluted. If you'd prefer, I'd be happy to extract it into something that's more manageable.

[coveralls]

Coverage remained the same at 100.0% when pulling b29308b on beaulac:datastore-exclude-array-primitives into 9b562af on GoogleCloudPlatform:master.

[stephenplusplus]

This looks good to me. I added the scenario into the system tests (the tests that run against the actual API).

Only problem now is that we're blocked for a short time while we are extracting the Datastore module into its own repo (soon to be https://github.com/googleapis/nodejs-datastore). After that, I'll port the PR over, and get it released ASAP.

Thank you for helping out!

[coveralls]

Coverage remained the same at 100.0% when pulling edb71c7 on beaulac:datastore-exclude-array-primitives into 9b562af on GoogleCloudPlatform:master.

[lostpebble]

Ah thank you @beaulac ! This has been a much wanted resolution.

[stephenplusplus]

@beaulac The other repo is now open: https://github.com/googleapis/nodejs-datastore -- I plan to port your work this week, but if you're willing, feel free to beat me to it! :)

[beaulac]

Merged in new repository.