Change Feed Estimator: Adds support for detailed estimation per lease in PREVIEW

[ealsur]

Description

This PR adds a new public API to obtain the detailed estimation per Lease on demand.
This API produces an iterator that returns pages of estimations. The iterator can have a customizable page size that will limit the number of estimations done at any given time. By default, it will estimate all leases on a single page.

In the Container, we have:

     public abstract ChangeFeedEstimator GetChangeFeedEstimator(
            string processorName,
            Container leaseContainer);

And the ChangeFeedEstimator:

public abstract class ChangeFeedEstimator
{
    public abstract FeedIterator<ChangeFeedProcessorState> GetCurrentStateIterator(ChangeFeedEstimatorRequestOptions?);
}

Where the ChangeFeedEstimatorRequestOptions gives an option to set the MaxItemCount:

public class ChangeFeedEstimatorRequestOptions
{
    public int MaxItemCount { get; set; }
}

Each page will contain a list of ChangeFeedProcessorState:

public class ChangeFeedProcessorState
{
    public string LeaseToken { get; }

    public long EstimatedLag { get; }

    public string InstanceName { get; }
}

Where the LeaseToken represents the identifier for that lease, the EstimatedLag is the estimation, and InstanceName is the name of the Change Feed processor instance processing that lease.

How can a user consume it

ChangeFeedEstimator estimator =  container.GetChangeFeedEstimator(processorName: "<processorNameUsedForProcessor>", leaseContainer);
FeedIterator<RemainingLeaseWork> iterator =  estimator.GetRemainingLeaseWorkIterator();

while (HasMoreResults)
{
    FeedResponse<RemainingLeaseWork> leaseWork = await iterator.ReadNextAsync(CancellationToken);
    foreach (RemainingLeaseWork estimation in leaseWork)
    {
        Console.WriteLine($"Lease {estimation.LeaseToken} owned by {estimation.InstanceName} has {estimation.RemainingWork} estimated pending items.");
    }
}

Code refactorings

The PR also refactors part of the CFP code. Originally there was code shared between the Processor, and the Estimator (when the Estimator was only a push model). Code refactoring had to be done to include the Estimator pull model and also maintain the Estimator push model (push now uses pull internally)

Also included some Code Analysis fixes on modified files plus new tests all around (emulator and unit tests).

Relevant files: ChangeFeedEstimatorIterator.cs, ContainerCore.Items.cs, ChangeFeedProcessorState.cs, ChangeFeedEstimatorRequestOptions.cs, ContractTests.cs.

Type of change

• New feature (non-breaking change which adds functionality)

Closing issues

Closes #1730

[ghost]

Closing due to in-activity, pease feel free to re-open.