Metrics.{counter,rateMeter}() shouldn't use TStream.pipe(); add TStream.peek(Peek)

Add `TStream.peek(Peek<T>)` and change `Metrics.counter(TStream)` and `Metrics.rateMeter(TStream)` to use it instead of `pipe()`.

The author of CounterOp and RateMeter implemented the functionality as Peek Oplets not as Consumer functions. Lacking a TStream.peek(Peek), TStream.pipe() must be used to add the Peek oplets to the topology.

The runtime treats TStream.peek(Consumer) generated Peek oplets rather differently than pipe related oplets (added via pipe() or indirectly via pipe-ish functional methods): see Connector.connect() vs Connector.peek(), and TStream.peek() returns "this" whereas the addition of pipe oplets returns a new TStream.

The use of pipe() in this case is partially responsible for the effect reported in QUARKS-189.

Adding TStream.peek(Peek) enables users/authors of Peek oplets to get the same peek-ish behavior as their functional peeker brethen. It continues to flesh out the general ability of API clients to implement and add oplets to the topology.

The growing number of "oplet" based analogs to the "function" based methods makes me wonder if the oplet ones should be broken out into another interface that TStream implements (`OpletTStream`?). It would contain the current `pipe(Pipe)`, `fanin(FanIn,List)`, `sink(Sink)`, and the new `peek(Peek)`, and any others that may be needed in the future - e.g., a `split(Split)` and/or one that can handle multiple iports and oports.

Instead, TStream.pipe() (ConnectorStream.pipe()) could be modified to deal with Pipe oplet args in the desired manner and document that Pipe oplets receive this special treatment and that pipe() returns "this" for them instead of a new TStream. Adding TStream.peek(Peek) seems like a clearer alternative, and perhaps oplet.core.Peek should not extend Pipe?