You can specify the following parameters for Availability metrics:

Collection Frequency 
Define the time interval (in minutes, hours, or days) at which a Simple Diagnostics Agent measures the metric. For example, if you set a collection frequency of one minute, the Simple Diagnostics Agent measures the metric once each minute.

Thresholds
A threshold defines when a metric receives a certain status, such as OK (green) or Critical (red). Availability metrics have four key performance indicators (KPIs). A threshold is available for each of these KPIs.

• Rating: This threshold is mandatory and is always switched on. This means it is always taken into account when calculating the metric status. The following settings are available:
  • Set to Red: Specifies a number of subsequent availability checks with an outcome Monitored object (for example, URL) is unavailable. When this number is reached, the metric status is set to red.
  • Set to Grey and Use Heartbeat: Specifies a number of subsequent collection intervals (for example, 1 minute) in which no availability check results are received by Health Monitoring. When this number is reached, the metric status is set to grey, and an alert is created if Use Heartbeat is selected. If Use Heartbeat is not selected no alert is created when the metric turns grey.
  • Back to Green: Specifies a number of subsequent successful availability checks. When this number is reached, the metric status turns from red or grey to green.

• Certificate Validity (HTTP Availability metrics only): This threshold is optional, which means you can switch it on or off. Only when this threshold is switched on is it taken into account when the metric status is calculated. The Certificate Validity is measured in days. 
  The following settings are available:
  • Warning: The metric status is set to yellow when the validity of the certificate of the monitored URL is less than or equal to the value specified in the Warning field.
  • Critical: The metric status is set to red when the validity of the certificate of the monitored URL is less than or equal to the value specified in the Critical field.

• Status Code: This threshold is optional, which means you can switch it on or off. Only when it is switched on is this threshold taken into account when the metric status is calculated. This threshold is available for all Availability metrics (HTTP Availability, RFC Availability, and TCP Availability).
  The following settings are available:
  • Warning: The metric status is set to yellow when the HTTP status code of the monitored object (for example, URL) is greater than or equal to the value specified in the Warning field.
  • Critical: The metric status is set to red when the HTTP status code of the monitored object (for example, URL) is greater than or equal to the value specified in the Critical field.

• Response Time: This threshold is optional, which means you can switch it on or off. Only when this threshold is switched on it is taken into account when calculating the metric status. This threshold is available for all Availability metrics (HTTP Availability, RFC Availability, and TCP Availability). The response time is measured in milliseconds.
  The following settings are available:
  • Warning: The metric status is set to yellow when the response time of the monitored object (for example, URL) is greater than or equal to the value specified in Warning field.
  • Critical: The metric status is set to red when the response time of the monitored object (for example, URL) is greater than or equal to the value specified in the Critical field.
  • Delay: Defines a time range in which the response time is evaluated. The unit for Delay is minute. 

Collection Frequency: 1 minute
Response Time: on
Warning: 2000
Critical: 4000
Delay: 10

Explanation
The metric status is set to yellow if the response time is greater than or equal to 2000 milliseconds but less than 4000 milliseconds 10 consecutive times. (10 = Delay/Collection Frequency)
The metric status is set to red if the response time is greater than or equal to 4000 milliseconds 10 consecutive times. (10 = Delay/Collection Frequency)

How is the metric status calculated when one or multiple thresholds are switched on?
The metric status is calculated according to the worst-case principle: If at least one of the activated thresholds or the Rating threshold returns red, the metric status is set to red.
If neither an activated threshold nor the Rating threshold returns red, but at least one of these thresholds returns yellow, the metric status is set to yellow.
If all activated thresholds and the Rating threshold return green, the metric status is set to green.

Alerts (optional)
If you don't maintain a threshold for a metric, an alert is generated when the metric status is set to red by the Health Monitoring app. If you maintain a threshold for a metric, an alert is generated when the defined threshold is reached. Alerts are displayed in the Health Monitoring and Alert Management applications.
The following alert settings are available:

• Alert Active
  Set the switch to On to generate an alert when the metric rating is set to red. Set to Off to prevent alerts being generated.
• Alert Name
  Specify a name for your alert. Be sure to use a descriptive name that others can easily understand.
• Severity: Select a severity value from 0 (Very Low) to 9 (Critical).
• Notification Variant: Select an alert consumer variant of the type Notification to be informed when the metric turns red (for example, by e‑mail).
• Outbound Connector Variant: Select an alert consumer variant of the type Outbound Integration to create a support ticket in a ticketing system, for example.
• Alert Documentation: Describe the alert and provide any additional information that the user may need.
• Alert Rules
  An alert rule selects metrics of the destination. Select metrics by either their name or their label.
  Use the Include table to select metrics for which alerts are created.
  Use the Exclude table to select metrics for which no alerts are created.

Availability Alerts Generated from Availability Metrics

Availability alerts can be generated from Availability (HTTP Availability, RFC Availability, and TCP Availability). A metric is reported to Service Availability Management as an unplanned outage if the following prerequisites are met:

• The metric has been assigned to a technical system.

An active service definition has been created for the technical system in Service Availability Management as explained here.