doc(metrics): CHANGELOG update and metrics documentation

Update CHANGELOG to publish that Firecracker now emits new metrics.
Also, document using a pseudo code how to extract metrics units
from Firecracker metrics names.

Signed-off-by: Sudan Landge <[email protected]>

Author: Sudan Landge

CHANGELOG.md

@@ -7,6 +7,14 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ## [Unreleased]
 
+### Added
+
+- [#4346](https://github.com/firecracker-microvm/firecracker/pull/4346):
+  Added support to emit aggregate (minimum/maximum/sum) latency for
+  `VcpuExit::MmioRead`, `VcpuExit::MmioWrite`, `VcpuExit::IoIn` and
+  `VcpuExit::IoOut`. The average for these VM exits is not emitted since
+  it can be deduced from the available emitted metrics.
+
 ## [v1.6.0]
 
 ### Added

docs/metrics.md

@@ -130,3 +130,20 @@ that component i.e. even if `vsock` device is not attached to the
 Microvm, Firecracker will still emit the Vsock metrics with key as
 `vsock` and value of all metrics defined in `VsockDeviceMetrics` as
 `0`.
+
+### Units for Firecracker metrics:
+
+Units for Firecracker metrics are embedded in their name.<br/>
+Below pseudo code should be to extract units from Firecracker metrics name:<br/>
+Note: An example of full_key for below logic is `"vcpu.exit_io_in_agg.min_us"`
+
+```
+    if substring "_bytes" or "_bytes_count" is present in any subkey of full_key
+        Unit is "Bytes"
+    else substring "_ms" is present in any subkey of full_key
+        Unit is "Milliseconds"
+    else substring "_us" is present in any subkey of full_key
+        Unit is "Microseconds"
+    else
+        Unit is "Count"
+```