<p>If there is a cookie set for a subdomain, metric.foo.com, is there a way for me to delete the metric.foo.com cookie on a request to www.foo.com? The browser (at least Firefox) seems to ignore a Set-Cookie with a domain of metric.foo.com.</p>

Cookies are only readable by the domain that created them, so if the cookie was created at metric.foo.com, it will have to be deleted under the same domain as it was created.  This includes sub-domains.
If you are required to delete a cookie from metric.foo.com, but are currently running a page at www.foo.com, you will not be able to.
In order to do this, you need to load the page from metric.foo.com, or create the cookie under foo.com so it can be accessable under any subdomain.  OR use this:
Response.cookies("mycookie").domain = ".foo.com"
...while creating it, AND before you delete it.
..untested - should work.