blob: Add support for key/value Metadata

[vangent]

Fixes #324.

The most controversial thing in this PR is force-lowercasing the Metadata keys during reading and writing. Provider behavior is inconsistent on this. I think both S3 and GCS store keys case-sensitive, but GCS claims it is case-insensitive

• GCP claims to be case-insensitive (search for "Field names are case-insensitive" here). However, the backend is actually case-sensitive. The JSON and XML-based APIs handle casing, but the HTTP one does not. Further discussion here.

• S3 claims to store them in lowercase (search for "user-defined metadata keys in lowercase" here). However, the HTTP API actually returns them capitalized due to Go's header canonicalization; see this long-standing issue complaining about it. It suggests parsing the headers yourself, but the original case can't be recovered that way so I'm not sure how that helps.

So, I think the best way to make the behavior consistent across providers and simple for users is to force-lowercase the metadata keys during reading and writing. There might still be unexpected behavior if:

1. The provider stores keys case-sensitive
2. The user is writing metadata using some mechanism other than Go Cloud
   In this scenario, multiple case-sensitive keys will be collapsed into a single key when reading using Go Cloud. Note that this would happen anyway for S3, at least when using the HTTP client.

[zombiezen]

Another reasonable restriction might be to keep keys as ASCII-clean for now.

[zombiezen]

Took a pass, mostly trying to poke around for edge cases about this, since it's tricky. I like this approach of just forcing everything to lowercase. I do suspect it will break things in surprising ways, but I suspect it to happen loudly.

[vangent]

PTAL.

[zombiezen]

LGTM. Please let @shantuo take a look before merging.

[shantuo]

LG, one question about driver comment.