zaptest: Extend ObservableLogs to support filtering

[prashantv]

In tests, it's useful to validate that a specific log exists with a specific field (similar to tchannel's log filtering).

I'm thinking of adding methods on ObservedLogs to filter:

• filter by message (exact or substring, not sure)
• filter by exact field match (key and value)
• filter by level

Imagining something like:

logger, sink := observer.New(...)
[...]
logs := sink.FilterMessage("Warning.").FilterField("serviceName", "foo")
assert.Equal(t, 1, logs.Len(), "Missing expected log in %v", sink)

cc @billf

[akshayjshah]

Sounds good to me.