Add URL query string quantization to Rack

[delner]

Adds quantization for Rack URL query string parameters. This behavior is activated by default, but can be configured with:

Datadog.configure do |c|
  # Default behavior: all values are quantized, fragment is removed.
  # http://example.com/path?category_id=1&sort_by=asc#featured --> http://example.com/path?category_id&sort_by
  # http://example.com/path?categories[]=1&categories[]=2 --> http://example.com/path?categories[]

  # Show values for any query string parameter matching 'category_id' exactly
  # http://example.com/path?category_id=1&sort_by=asc#featured --> http://example.com/path?category_id=1&sort_by
  c.use :rack, quantize: { query: { show: ['category_id'] } }

  # Show all values for all query string parameters
  # http://example.com/path?category_id=1&sort_by=asc#featured --> http://example.com/path?category_id=1&sort_by=asc
  c.use :rack, quantize: { query: { show: :all } }

  # Totally exclude any query string parameter matching 'sort_by' exactly
  # http://example.com/path?category_id=1&sort_by=asc#featured --> http://example.com/path?category_id
  c.use :rack, quantize: { query: { exclude: ['sort_by'] } }

  # Remove the query string entirely
  # http://example.com/path?category_id=1&sort_by=asc#featured --> http://example.com/path
  c.use :rack, quantize: { query: { exclude: :all } }

  # Show URL fragments
  # http://example.com/path?category_id=1&sort_by=asc#featured --> http://example.com/path?category_id&sort_by#featured
  c.use :rack, quantize: { fragment: :show }
end

[palazzem]

couple of things, but I think it's great in the overall!

[palazzem]

nice write up!

[palazzem]

specific

[delner]

facepalm

[palazzem]

can we handle any kind of exception here? since it's the first middleware, in case of an issue (i.e. encoding/decoding/filtering problem) would be great to avoid propagating our internal exception. I think it's enough to handle the exception somewhere, and log it as debug().

[delner]

@palazzem are you talking about this tagging function in general? Or specifically the use of quantization here?

[palazzem]

since it's a free function and can be used somewhere else, can we add a test without using URL encoding? so a query string such as value=繋がってて.

[delner]

Great idea. I'll add in some more edge case tests around encoding and Unicode characters.

[delner]

This feature is dependent on #384 and should be rebased and merged after that PR is merged.

Do we want to consider this a breaking change? Requires no action on the user's part, but will change their URLs/query strings.

[palazzem]

@delner not really since it doesn't impact the API or anything else. I think it's enough to mention this in our migration guide just to let them know the new behavior. Metrics and anything else is not impacted.