[hostmetrics] change cpu.load.average metric units (open-telemetry#29915

)

**Description:** 
The system.cpu.load_average.5m uses 1 as unit, which is reserved for
utilization metrics -
https://opentelemetry.io/docs/specs/semconv/general/metrics/#instrument-units

CPU Load Average does not track utilization and can go above 

**Link to tracking Issue:** open-telemetry#29914

.chloggen/host-metrics-units.yaml

@@ -0,0 +1,27 @@
+# Use this changelog template to create an entry for release notes.
+
+# One of 'breaking', 'deprecation', 'new_component', 'enhancement', 'bug_fix'
+change_type: bug_fix
+
+# The name of the component, or a single word describing the area of concern, (e.g. filelogreceiver)
+component: hosmetricsreceiver
+
+# A brief description of the change.  Surround your text with quotes ("") if it needs to start with a backtick (`).
+note: "change cpu.load.average metrics from 1 to {thread}"
+
+# Mandatory: One or more tracking issues related to the change. You can use the PR number here if no issue exists.
+issues: [29914]
+
+# (Optional) One or more lines of additional information to render under the primary note.
+# These lines will be padded with 2 spaces and then inserted directly into the document.
+# Use pipe (|) for multiline entries.
+subtext:
+
+# If your change doesn't affect end users or the exported elements of any package,
+# you should instead start your pull request title with [chore] or use the "Skip Changelog" label.
+# Optional: The change log or logs in which this entry should be included.
+# e.g. '[user]' or '[user, api]'
+# Include 'user' if the change is relevant to end users.
+# Include 'api' if there is a change to a library API.
+# Default: '[user]'
+change_logs: [user]

receiver/hostmetricsreceiver/internal/scraper/loadscraper/documentation.md

@@ -20,20 +20,20 @@ Average CPU Load over 15 minutes.
 
 | Unit | Metric Type | Value Type |
 | ---- | ----------- | ---------- |
-| 1 | Gauge | Double |
+| {thread} | Gauge | Double |
 
 ### system.cpu.load_average.1m
 
 Average CPU Load over 1 minute.
 
 | Unit | Metric Type | Value Type |
 | ---- | ----------- | ---------- |
-| 1 | Gauge | Double |
+| {thread} | Gauge | Double |
 
 ### system.cpu.load_average.5m
 
 Average CPU Load over 5 minutes.
 
 | Unit | Metric Type | Value Type |
 | ---- | ----------- | ---------- |
-| 1 | Gauge | Double |
+| {thread} | Gauge | Double |

receiver/hostmetricsreceiver/internal/scraper/loadscraper/metadata.yaml

@@ -8,20 +8,20 @@ metrics:
   system.cpu.load_average.1m:
     enabled: true
     description: Average CPU Load over 1 minute.
-    unit: 1
+    unit: "{thread}"
     gauge:
       value_type: double
 
   system.cpu.load_average.5m:
     enabled: true
     description: Average CPU Load over 5 minutes.
-    unit: 1
+    unit: "{thread}"
     gauge:
       value_type: double
 
   system.cpu.load_average.15m:
     enabled: true
     description: Average CPU Load over 15 minutes.
-    unit: 1
+    unit: "{thread}"
     gauge:
       value_type: double