[core][wmi] WMI module + CounterType calculators

A lightweight Python WMI module wrapper built on top of `pywin32` and
`win32com` extensions.

**Specifications**
* Based on top of the `pywin32` and `win32com` third party extensions
* only
* Compatible with `Raw`* and `Formatted` Performance Data classes
    * Dynamically resolve properties' counter types
    * Hold the previous/current `Raw` samples to compute/format new
    * values*
* Fast and lightweight
    * Avoid queries overhead
    * Cache connections and qualifiers
    * Use `wbemFlagForwardOnly` flag to improve enumeration/memory
    * performance

*\* `Raw` data formatting relies on the avaibility of the corresponding
calculator.
Please refer to `checks.lib.wmi.counter_type` for more information*

Original discussion thread:
#1952
Credits to @TheCloudlessSky (https://github.com/TheCloudlessSky)
"""

checks/libs/wmi/counter_type.py

@@ -0,0 +1,139 @@
+"""
+Implementation of WMI calculators a few `CounterType`(s).
+
+
+Protocol.
+
+    Use MSDN to lookup the class, property and counter type to determine the
+    appropriate calculator.
+
+    For example, "Win32_PerfRawData_PerfOS_Memory" is located at:
+      https://msdn.microsoft.com/en-us/library/aa394314(v=vs.85).aspx
+    and the "CacheBytes" property has a CounterType = 65792 which
+    can be determined from:
+      https://msdn.microsoft.com/en-us/library/aa389383(v=vs.85).aspx
+    The CounterType 65792 (PERF_COUNTER_LARGE_RAWCOUNT) is defined here:
+      https://technet.microsoft.com/en-us/library/cc780300(v=ws.10).aspx
+
+    From: https://msdn.microsoft.com/en-us/library/aa389383(v=vs.85).aspx
+
+Original discussion thread: https://github.com/DataDog/dd-agent/issues/1952
+Credits to @TheCloudlessSky (https://github.com/TheCloudlessSky)
+"""
+
+_counter_type_calculators = {}
+
+
+class UndefinedCalculator(Exception):
+    """
+    No calculator is defined for the given CounterType.
+    """
+    pass
+
+
+def calculator(counter_type):
+    """
+    A decorator that assign a counter_type to its calculator.
+    """
+    def set_calculator(func):
+        _counter_type_calculators[counter_type] = func
+        return func
+    return set_calculator
+
+
+def get_calculator(counter_type):
+    """
+    Return the calculator associated with the counter_type when it exists.
+
+    Raise a UndefinedCalculator exception otherwise.
+    """
+    try:
+        return _counter_type_calculators[counter_type]
+    except KeyError:
+        raise UndefinedCalculator
+
+
+def get_raw(previous, current, property_name):
+    """
+    Returns the vanilla RAW property value.
+
+    Not associated with any counter_type. Used to fallback when no calculator
+    is defined for a given counter_type.
+    """
+    return current[property_name]
+
+
+@calculator(65536)
+def calculate_perf_counter_rawcount(previous, current, property_name):
+    """
+    PERF_COUNTER_RAWCOUNT
+
+    https://technet.microsoft.com/en-us/library/cc757032(v=ws.10).aspx
+    """
+    return current[property_name]
+
+
+@calculator(65792)
+def calculate_perf_counter_large_rawcount(previous, current, property_name):
+    """
+    PERF_COUNTER_LARGE_RAWCOUNT
+
+    https://technet.microsoft.com/en-us/library/cc780300(v=ws.10).aspx
+    """
+    return current[property_name]
+
+
+@calculator(542180608)
+def calculate_perf_100nsec_timer(previous, current, property_name):
+    """
+    PERF_100NSEC_TIMER
+
+    https://technet.microsoft.com/en-us/library/cc728274(v=ws.10).aspx
+    """
+    n0 = previous[property_name]
+    n1 = current[property_name]
+    d0 = previous["Timestamp_Sys100NS"]
+    d1 = current["Timestamp_Sys100NS"]
+
+    if n0 is None or n1 is None:
+        return
+
+    return (n1 - n0) / (d1 - d0) * 100
+
+
+@calculator(272696576)
+def calculate_perf_counter_bulk_count(previous, current, property_name):
+    """
+    PERF_COUNTER_BULK_COUNT
+
+    https://technet.microsoft.com/en-us/library/cc757486(v=ws.10).aspx
+    """
+    n0 = previous[property_name]
+    n1 = current[property_name]
+    d0 = previous["Timestamp_Sys100NS"]
+    d1 = current["Timestamp_Sys100NS"]
+    f = current["Frequency_Sys100NS"]
+
+    if n0 is None or n1 is None:
+        return
+
+    return (n1 - n0) / ((d1 - d0) / f)
+
+
+@calculator(272696320)
+def calculate_perf_counter_counter(previous, current, property_name):
+    """
+    PERF_COUNTER_COUNTER
+
+    https://technet.microsoft.com/en-us/library/cc740048(v=ws.10).aspx
+    """
+    n0 = previous[property_name]
+    n1 = current[property_name]
+    d0 = previous["Timestamp_Sys100NS"]
+    d1 = current["Timestamp_Sys100NS"]
+    f = current["Frequency_Sys100NS"]
+
+    if n0 is None or n1 is None:
+        return
+
+    return (n1 - n0) / ((d1 - d0) / f)