contrib/bradfitz/gomemcache: add memcache tracer

[dd-caleb]

Fixes #290

This adds a memcache trace library. We wrap the memcache.Client and return a new struct with all the same methods:

type Client struct {
	*memcache.Client
	cfg     *clientConfig
	context context.Context
}

func (c *Client) Add(item *memcache.Item) error {
	span := c.startSpan("Add")
	err := c.Client.Add(item)
	span.Finish(tracer.WithError(err))
	return err
}

We also add one additional method, WithContext:

func (c *Client) WithContext(ctx context.Context) *Client {
	// the existing memcache client doesn't support context, but may in the
	// future, so we do a runtime check to detect this
	mc := c.Client
	if hasWithContext, ok := (interface{})(c.Client).(interface {
		WithContext(context.Context) *memcache.Client
	}); ok {
		mc = hasWithContext.WithContext(ctx)
	}
	return &Client{
		Client:  mc,
		cfg:     c.cfg,
		context: ctx,
	}
}

This is similar to how the redis library implements Context.

[gbbr]

An initial round of comments.

[gbbr]

We usually defer this in most examples.

[gbbr]

Can we make up some better imaginary names than "example" for span and service? Let's think of a basic simplistic use-case or "hello world" example.

[gbbr]

This (interface{}) hack is nasty. But it's ok, it's a good approach.

[gbbr]

hasWithContext sounds like a boolean. Can we use something else (some examples: v, wc)

[gbbr]

tracer.SpanType(ext.SpanTypeMemcached)

[gbbr]

Can we move this to a separate function and have a unit test for it?

[gbbr]

please add an empty line between

[gbbr]

We should write a test for this helper. Our main tests rely on it.

[gbbr]

Thanks!

[gbbr]

You might have already noticed, but in case not, we're using resource_class: xlarge. I'm not sure how long that'll last us if we keep adding new services. It's 8 CPUs and 16GB.

[gbbr]

👍