Add histogram context manager and decorator

[oldpatricka]

This MR adds support for a new decorator for histograms, much the same as the distributed decorator. (It is heavily cribbed from same).

A decorator or context manager that will measure the histogram of a function's/context's run time using custom metric HISTOGRAM.

E.g. of usage follows:

@statsd.histogrammed('user.query.time', sample_rate=0.5)
def get_user(user_id):
    # Do what you need to ...
    pass

# Is equivalent to ...
with statsd.histogrammed('user.query.time', sample_rate=0.5):
    # Do what you need to ...
    pass

# Is equivalent to ...
start = time.time()
try:
    get_user(user_id)
finally:
    statsd.histogram('user.query.time', time.time() - start)

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What does this PR do?

This adds a new histogrammed decorator that works much the same way as timed and distributed. I wanted to time a function using a histogram in my project, and added very similar code to my project to do so.

This implements #770

Description of the Change

This copies what is done with the distributed decorator extremely closely.

Alternate Designs

This is a simple change so no alternatives were considered.

Possible Drawbacks

I can't imagine any.

Verification Process

I ran a forked of the dogstatsd client against my datadog account and saw metrics appear as expected.

Additional Notes

N/A

Release Notes

N/A

Review checklist (to be filled by reviewers)

• Feature or bug fix MUST have appropriate tests (unit, integration, etc...)
• PR title must be written as a CHANGELOG entry (see why)
• Files changes must correspond to the primary purpose of the PR as described in the title (small unrelated changes should have their own PR)
• PR must have one changelog/ label attached. If applicable it should have the backward-incompatible label attached.
• PR should not have do-not-merge/ label attached.
• If Applicable, issue must have kind/ and severity/ labels attached at least.

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