Fix verb / target for 'Metric.create' / 'Sink.create'.

[tseaver]

• Spec mandates 'POST' to the container, rather than 'PUT' to the putative URL of the entity itself.
• Note that metric creation worked before; sink creation is blocked due to permission issues (see Permissions problem trying to create a sink #1614).

[tseaver]

@dhermes PTAL. System tests which create metrics work both ways; a new system test for creating sinks fails (see #1614).

[dhermes]

How do you mean "worked before"? Also, why the change needed? Was the previous implementation not based on the docs?

[tseaver]

How do you mean "worked before"?

The system tests for #1610, #1611, #1612, #1613 were all running cleanly using PUT to the not-yet-existing metric's URI.

Also, why the change needed? Was the previous implementation not based on the docs?

I misread the docs, because I was looking at the create wrappers for otther APIs (pubsub, search) which use PUT to create resources.

[dhermes]

LGTM