[Infra] Add a link to host metrics docs (#190993)

## Summary

The link is not shown for K8s pods

<img width="500px" alt="image"
src="https:/user-attachments/assets/5c86d44e-4088-4bd6-a151-3fcf518dc752">

The link is shown for hosts

<img width="500px" alt="image"
src="https:/user-attachments/assets/7b195990-1b28-4e73-818e-3024a0725007">



### How to test

- Navigate to `Inventory` 
- Click on Alerts and Rules > Infrastructure > Create Inventory rule
- Click on the metric to open the metric selection popover
  - Validate if the link appears and redirects to the docs page
- Do the same with other asset types (Containers, Docker, etc)

x-pack/plugins/observability_solution/infra/public/alerting/inventory/components/expression.tsx

@@ -444,7 +444,8 @@ const StyledHealthCss = css`
 export const ExpressionRow: FC<PropsWithChildren<ExpressionRowProps>> = (props) => {
  const [isExpanded, toggle] = useToggle(true);
 
- const { children, setRuleParams, expression, errors, expressionId, remove, canDelete } = props;
+ const { children, setRuleParams, expression, errors, expressionId, remove, canDelete, nodeType } =
+ props;
  const {
  metric,
  comparator = COMPARATORS.GREATER_THAN,
@@ -554,7 +555,7 @@ export const ExpressionRow: FC<PropsWithChildren<ExpressionRowProps>> = (props)
  const ofFields = useMemo(() => {
  let myMetrics: SnapshotMetricType[] = hostSnapshotMetricTypes;
 
- switch (props.nodeType) {
+ switch (nodeType) {
  case 'awsEC2':
  myMetrics = awsEC2SnapshotMetricTypes;
  break;
@@ -577,8 +578,8 @@ export const ExpressionRow: FC<PropsWithChildren<ExpressionRowProps>> = (props)
  myMetrics = containerSnapshotMetricTypes;
  break;
  }
- return myMetrics.map((myMetric) => toMetricOpt(myMetric, props.nodeType));
- }, [props.nodeType]);
+ return myMetrics.map((myMetric) => toMetricOpt(myMetric, nodeType));
+ }, [nodeType]);
 
  return (
  <>
@@ -608,6 +609,7 @@ export const ExpressionRow: FC<PropsWithChildren<ExpressionRowProps>> = (props)
  text: string;
  }>
  }
+ nodeType={nodeType}
  onChange={updateMetric}
  onChangeCustom={updateCustomMetric}
  errors={errors}

x-pack/plugins/observability_solution/infra/public/alerting/inventory/components/metric.tsx

@@ -13,6 +13,7 @@ import {
  EuiFlexGroup,
  EuiFlexItem,
  EuiFormRow,
+ EuiLink,
  EuiPopover,
  EuiPopoverTitle,
  EuiSelect,
@@ -24,6 +25,8 @@ import { FormattedMessage } from '@kbn/i18n-react';
 import { debounce } from 'lodash';
 import React, { useCallback, useMemo, useState } from 'react';
 import { IErrorObject } from '@kbn/triggers-actions-ui-plugin/public';
+import type { InventoryItemType } from '@kbn/metrics-data-access-plugin/common';
+import { HOST_METRICS_DOC_HREF } from '../../../common/visualizations';
 import { useMetricsDataViewContext } from '../../../containers/metrics_source';
 import { getCustomMetricLabel } from '../../../../common/formatters/get_custom_metric_label';
 import {
@@ -37,6 +40,7 @@ import {
 interface Props {
  metric?: { value: string; text: string };
  metrics: Array<{ value: string; text: string }>;
+ nodeType: InventoryItemType;
  errors: IErrorObject;
  onChange: (metric?: string) => void;
  onChangeCustom: (customMetric?: SnapshotCustomMetricInput) => void;
@@ -91,6 +95,7 @@ export const MetricExpression = ({
  onChange,
  onChangeCustom,
  popupPosition,
+ nodeType,
 }: Props) => {
  const [popoverOpen, setPopoverOpen] = useState(false);
  const [customMetricTabOpen, setCustomMetricTabOpen] = useState(metric?.value === 'custom');
@@ -312,7 +317,23 @@ export const MetricExpression = ({
  </>
  ) : (
  <EuiFormRow fullWidth>
- <EuiFlexGroup>
+ <EuiFlexGroup direction="column" gutterSize="s">
+ {nodeType === 'host' && (
+ <EuiFlexItem>
+ <EuiLink
+ data-test-subj="alertFlyoutHostMetricsDocumentationLink"
+ href={HOST_METRICS_DOC_HREF}
+ target="_blank"
+ >
+ {i18n.translate(
+ 'xpack.infra.metrics.alertFlyout.expression.metric.whatAreTheseMetricsLink',
+ {
+ defaultMessage: 'What are these metrics?',
+ }
+ )}
+ </EuiLink>
+ </EuiFlexItem>
+ )}
  <EuiFlexItem className="actOf__metricContainer">
  <EuiComboBox
  fullWidth