introducing view

[as-polyakov]

• View class as an overarching concept for AggregationFactory. In the future we can add more specifics into it
• The grand idea here is basically based around OpenCensus Views. We want to be able to register custom projections (views) on the metrics which would allow us to use different aggregations and pre-processing techniques.
• This is the very first step where we just introduce a view and wrap existing AggregatorFacotry
• Immediately next we would like to have LabelsProcessors added to the views which are going to be a separate PR, but at a glance - same thing we discussed in MetricsProcessor #1810. Whole PR might be found here introduction of View and first version of LabelsProessor #2800

[jkwatson]

Can you give any hints about what you intend to add to Views? Maybe a design document with some ideas? I'm all on board with supporting opencensus style views, but I'd like to understand where we're going with the approach in this PR.

[bogdandrutu]

@jkwatson I think the next steps are in #1810

[jkwatson]

@jkwatson I think the next steps are in #1810

We should just be clear at this point that this is all very prototype work. This isn't in any specification yet, and is extremely likely to be re-worked before metrics get to 1.0.