Add configurable user agent prefix flag for mount-s3

[dannycjones]

Description of change

If mountpoint-s3 is run as part of a larger application, user/customer may want to configure a prefix for the user agent to indicate S3 requests came from that particular application.

This change adds a new --user-agent-prefix <PREFIX> flag to achieve this.

Relevant issues: #546

Does this change impact existing behavior?

No change to existing behaviour. Users can now add a prefix to the HTTP 'User-Agent' header.

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[dannycjones]

Tested this locally by pointing mount-s3 at a local HTTP server. Looks good.

[sauraank]

LGTM.
We just need to see why the tests are failing. The ASAN failure did not repeat when I repeat the Address sanitizer job on my PR. But, both the tests are failing in other PRs as well.

[dannycjones]

This change should update the changelog: https://github.com/awslabs/mountpoint-s3/blob/main/mountpoint-s3/CHANGELOG.md

I'd like to update it before merging this.

[dannycjones]

This will conflict with #556. Prefer to merge in #556, rebase this, and merge.

[jamesbornholt]

I kind of think this should be an undocumented option. It's really only useful for us right now, right? Not sure it helps customers for it to show up in --help. But it would be our first undocumented option. Maybe an alternative is a new help section for "advanced options" or something, to make clear this isn't something most customers who aren't AWS would ever need to use.

[dannycjones]

I was thinking it may be of value if third-parties wanted to include mount-s3 in their project/product, and include the user agent in their request.

I like the idea of moving it under an advanced options, so I'll update with that.

[dannycjones]

I'm actually less comfortable with moving it into a new "Advanced options", since someone looking for this kind of option would expect it to be with "Client options". Our intention is also for things like --max-threads <> and --part-size <> to be things most customers don't need to configure either.

At the moment, it shows up at the top of the list (since its based on the order it is defined). How do you feel about moving it to the bottom of the "Client options" list like below?

Client options:
      --maximum-throughput-gbps <N>  Maximum throughput in Gbps [default: auto-detected on EC2 instances, 10 Gbps elsewhere]
      --max-threads <N>              Maximum number of FUSE daemon threads [default: 16]
      --part-size <PART_SIZE>        Part size for multi-part GET and PUT [default: 8388608]
      --user-agent-prefix <PREFIX>   Configure a string to be prepended to the 'User-Agent' HTTP request header for all S3 requests

[jamesbornholt]

Is anyone really going to be looking for it? Only AWS really gets to see the user agent. Unlike the other options, this is something a customer would basically never configure — it's really only useful for bundling Mountpoint into something else (and that basically means only useful for us).

[dannycjones]

OK, let me move it to an "Advanced options" section at the bottom.