in_mem_metrics: initial documentation.

This adds documentation for the new `in_mem_metrics` plugin: fluent/fluent-bit#7615.

Signed-off-by: Phillip Whelan <[email protected]>

Author: pwhelan

pipeline/inputs/mem-metrics.md

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+# Memory Metrics (mem_metrics)
+
+The **mem_metrics**, or memory metrics plugin, generates metrics for memory statistics for processes using the `smaps_rollup` file for each process in /proc. At the moment this plugin works exclusively on Linux.
+
+# Configuration Parameters
+
+| **Key***      | Description                                        | Default      |
+| :------------ | :------------------------------------------------- | :----------- |
+| proc_path     | The path of the proc pseudo filesystem             | /proc        |
+| filter_exec   | Filter for a single executable by path             | **inactive** |
+| filter_cmd    | Filter by command line                             | **inactive** |
+| filter_pid    | Filter by comma delimited list of PIDs             | **inactive** |
+| interval_sec  | Set the interval seconds between events generation | 5            |
+| interval_nsec | Set the nanoseconds interval (sub seconds)         | 0            |
+
+The `filter_pid` can include or be set to either `0` or `self` to refer to the fluent-bit process itself.
+
+## Getting Started
+
+You can run the plugin from the command line or through a configuration file. By default metrics will be generated for all processes the current user can analyze.
+
+### Command Line
+
+```bash
+$ fluent-bit -i mem_metrics -o stdout
+Fluent Bit v2.1.8
+* Copyright (C) 2015-2022 The Fluent Bit Authors
+* Fluent Bit is a CNCF sub-project under the umbrella of Fluentd
+* https://fluentbit.io
+
+2023-07-26T14:37:49.919778443Z node_smaps_rollup_rss{pid="4540"} = 21064
+2023-07-26T14:37:49.919778443Z node_smaps_rollup_pss{pid="4540",type="clean"} = 9598
+2023-07-26T14:37:49.919778443Z node_smaps_rollup_pss{pid="4540",type="dirty"} = 5998
+2023-07-26T14:37:49.919778443Z node_smaps_rollup_pss{pid="4540",type="anon"} = 5996
+2023-07-26T14:37:49.919778443Z node_smaps_rollup_pss{pid="4540",type="file"} = 3602
+2023-07-26T14:37:49.919778443Z node_smaps_rollup_pss{pid="4540",type="shmem"} = 0
+2023-07-26T14:37:49.919778443Z node_smaps_rollup_shared{pid="4540",type="clean"} = 15008
+2023-07-26T14:37:49.919778443Z node_smaps_rollup_shared{pid="4540",type="dirty"} = 4
+2023-07-26T14:37:49.919778443Z node_smaps_rollup_private{pid="4540",type="clean"} = 56
+2023-07-26T14:37:49.919778443Z node_smaps_rollup_private{pid="4540",type="dirty"} = 5996
+2023-07-26T14:37:49.919778443Z node_smaps_rollup_referenced{pid="4540"} = 21064
+2023-07-26T14:37:49.919778443Z node_smaps_rollup_anonymous{pid="4540"} = 5996
+2023-07-26T14:37:49.919778443Z node_smaps_rollup_lazy_free{pid="4540"} = 0
+2023-07-26T14:37:49.919778443Z node_smaps_rollup_anon_huge_pages{pid="4540"} = 0
+2023-07-26T14:37:49.919778443Z node_smaps_rollup_shmem_pmd_mapped{pid="4540"} = 0
+2023-07-26T14:37:49.919778443Z node_smaps_rollup_file_pmd_mapped{pid="4540"} = 0
+2023-07-26T14:37:49.919778443Z node_smaps_rollup_shared_hugetlb{pid="4540"} = 0
+2023-07-26T14:37:49.919778443Z node_smaps_rollup_private_hugetlb{pid="4540"} = 0
+2023-07-26T14:37:49.919778443Z node_smaps_rollup_swap{pid="4540"} = 0
+2023-07-26T14:37:49.919778443Z node_smaps_rollup_swap_pss{pid="4540"} = 0
+2023-07-26T14:37:49.919778443Z node_smaps_rollup_locked{pid="4540"} = 0
+```
+
+### Configuration File
+
+Minimal pipeline for logging memory metrics to the console:
+
+```ini
+[INPUT]
+    Name mem_metrics
+    Tag  mem
+
+[OUTPUT]
+    Name  stdout
+    Match mem
+```
+
+Log all instances of fluent-bit to prometheus:
+
+```ini
+[INPUT]
+    Name       mem_metrics
+    Filter_cmd fluent-bit
+    Tag        mem
+
+[OUTPUT]
+    Name                 prometheus_remote_write
+    Match                mem
+    Host                 localhost
+    Port                 9090
+    Uri                  /prometheus/v1/write?prometheus_server=mem
+```
+
+### Exposed Metrics
+
+All metrics are logged as gauges since they can both increase and decrease. Currenrtly supported gauges are:
+  * node_smaps_rollup_rss
+  * node_smaps_rollup_pss
+    * type=clean
+    * type=dirty
+    * type=anon
+    * type=file
+    * type=shmem
+  *  node_smaps_rollup_shared
+    * type=clean
+    * type=dirty
+  *  node_smaps_rollup_private
+    * type=dirty
+    * type=clean
+  * node_smaps_rollup_referenced
+  * node_smaps_rollup_anonymous
+  * node_smaps_rollup_lazy_free
+  * node_smaps_rollup_anon_huge_pages
+  * node_smaps_rollup_shmem_pmd_mapped
+  * node_smaps_rollup_file_pmd_mapped
+  * node_smaps_rollup_shared_hugetlb
+  * node_smaps_rollup_private_hugetlb
+  * node_smaps_rollup_swap
+  * node_smaps_rollup_swap_pss
+  * node_smaps_rollup_locked