[v2,3/3] doc: add howto guide for lcore poll busyness

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+..  SPDX-License-Identifier: BSD-3-Clause
+    Copyright(c) 2022 Intel Corporation.
+
+Lcore Poll Busyness Telemetry
+========================
+
+The lcore poll busyness telemetry provides a built-in, generic method of gathering
+lcore utilization metrics for running applications. These metrics are exposed
+via a new telemetry endpoint.
+
+Since most DPDK APIs poll for packets, the poll busyness is calculated based on
+APIs receiving packets. Empty polls are considered as idle, while non-empty polls
+are considered busy. Using the amount of cycles spent processing empty polls, the
+busyness can be calculated and recorded.
+
+Application Specified Busyness
+------------------------------
+
+Improved accuracy of the reported busyness may need more contextual awareness
+from the application. For example, a pipelined application may make a number of
+calls to rx_burst before processing packets. Any processing done on this 'bulk'
+would need to be marked as "busy" cycles, not just the last received burst. This
+type of awareness is only available within the application.
+
+Applications can be modified to incorporate the extra contextual awareness in
+order to improve the reported busyness by marking areas of code as "busy" or
+"idle" appropriately. This can be done by inserting the timestamping macro::
+
+    RTE_LCORE_TELEMETRY_TIMESTAMP(0)    /* to mark section as idle */
+    RTE_LCORE_TELEMETRY_TIMESTAMP(32)   /* where 32 is nb_pkts to mark section as busy (non-zero is busy) */
+
+All cycles since the last state change will be counted towards the current state's
+counter.
+
+Consuming the Telemetry
+-----------------------
+
+The telemetry gathered for lcore poll busyness can be read from the `telemetry.py`
+script via the new `/eal/lcore/poll_busyness` endpoint::
+
+    $ ./usertools/dpdk-telemetry.py
+    --> /eal/lcore/poll_busyness
+    {"/eal/lcore/poll_busyness": {"12": -1, "13": 85, "14": 84}}
+
+* Cores not collecting busyness will report "-1". E.g. control cores or inactive cores.
+* All enabled cores will report their busyness in the range 0-100.
+
+Disabling Lcore Poll Busyness Telemetry
+----------------------------------
+
+Some applications may not want lcore poll busyness telemetry to be tracked, for
+example performance critical applications or applications that are already being
+monitored by other tools gathering similar or more application specific information.
+
+For those applications, there are two ways in which this telemetry can be disabled.
+
+At compile time
+^^^^^^^^^^^^^^^
+
+Support can be disabled at compile time via the meson option. It is enabled by
+default.::
+
+    $ meson configure -Denable_lcore_poll_busyness=false
+
+At run time
+^^^^^^^^^^^
+
+Support can also be disabled during runtime. This comes at the cost of an
+additional branch, however no additional function calls are performed.
+
+To disable support at runtime, a call can be made to the
+`/eal/lcore/poll_busyness_disable` endpoint::
+
+    $ ./usertools/dpdk-telemetry.py
+    --> /eal/lcore/poll_busyness_disable
+    {"/eal/lcore/poll_busyness_disable": {"busyness_enabled": 0}}
+
+It can be re-enabled at run time with the `/eal/lcore/poll_busyness_enable`
+endpoint.