feat: Introducing the Level Trigger Mode

[haoel]

The PR introduces the level trigger mode with three types of altering intervals for continuing DOWN status:

1. Regular Strategy: Notifications are sent at the same frequency as the probe time, continuing until the maximum number of notifications is reached.

   the interval can be configured by factor.
         interval = factor
   if the factor = 1, then the alert would be sent at the failure of 1, 2，3，4，5，6，7...
                      the interval is 1, 1, 1, 1, 1, 1, 1...
   if the factor = 2, then the alert would be sent at the failure of 1, 3, 5, 7, 9, 11, 13...
                      the interval is 2, 2, 2, 2, 2, 2, 2...
   if the factor = 3, then the alert would be sent at the failure of 1, 4, 7, 10, 13, 16, 19...
                      the interval is 3, 3, 3, 3, 3, 3, 3...
   

2. Incremental Strategy: Notifications are sent at increasing intervals, continuing until the maximum number of notifications is reached. With this strategy, notifications are sent at the failure of 1, 2, 4, 7, 11, 16, 22, and so on. the intervals are 1, 2, 3, 4, 5, 6...

   the interval is increased linearly.
   
        interval = factor * ( failure times - 1 ) + 1
   
   if the factor = 1, then the alert would be sent at the failure of 1, 2, 4, 7, 11, 16, 22, 29, 37...
                      the interval is 1, 2, 3, 4, 5, 6, 7, 8, 9...
    if the factor = 2, then the alert would be sent at the failure of 1, 3, 7, 13, 21, 31, 43, 57, 73...
                      the interval is 2, 4, 6, 8, 10, 12, 14, 16, 18...
    if the factor = 3, then the alert would be sent at the failure of 1, 4, 10, 19, 31, 46, 64, 85, 109...
                      the interval is 3, 6, 9, 12, 15, 18, 21, 24, 27...
   

3. Exponential Strategy: Notifications are sent at exponentially increasing intervals, continuing until the maximum number of notifications is reached. With this strategy, notifications are sent at the failure of 1, 2, 4, 8, 16, 32, and so on.

   the interval is increased exponentially.
   
       interval =   failure times + factor * ( failure times - 1 )
   
   if the factor = 1, then the alert would be sent at the failure of 1, 2, 4, 8, 16, 32, 64, 128, 256...
                       the interval is 1, 2, 4, 8, 16, 32, 64, 128...
   if the factor = 2, then the alert would be sent at the failure of 1, 3, 9, 27, 81, 243, 729, 2187, 6561...
                      the interval is 2, 6, 18, 54, 162, 486, 1458, 4374, 13122...
   if the factor = 3, then the alert would be sent at the failure of 1, 4, 16, 64, 256, 1024...
                      the interval is 3, 12, 48, 192, 768...
   
   
   

Note:

The default strategy is regular and the max time is 1, this would be the same as the old behavior - edge trigger.

The configuration can be put into any probe or global setting.

probe:
    http: dummy
    url: http://example.com/
    alert:
        strategy: "regular"   # can be "regular", "increment" & "exponentiation", default is regular
        factor: 1 # the factor impact the alert interval
        max: 10     # the max times to send a notification if the endpoint is continuously down

settings:
   probe:
       alert:
           strategy: "regular"   # can be "regular", "increment" & "exponentiation", default is regular
           factor: 1 # the factor impact the alert interval
           max: 10     # the max times to send a notification if the endpoint is continuously down

Close #314

[codecov-commenter]

Codecov Report

Patch coverage: 100.00% and no project coverage change.

Comparison is base (682462a) 99.67% compared to head (1bb6b5b) 99.67%.

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Additional details and impacted files

@@           Coverage Diff            @@
##             main     #315    +/-   ##
========================================
  Coverage   99.67%   99.67%            
========================================
  Files          82       83     +1     
  Lines        5503     5618   +115     
========================================
+ Hits         5485     5600   +115     
  Misses         12       12            
  Partials        6        6            

Impacted Files	Coverage Δ
conf/conf.go	96.73% <ø> (ø)
global/global.go	100.00% <ø> (ø)
channel/channel.go	100.00% <100.00%> (ø)
global/probe.go	100.00% <100.00%> (ø)
probe/base/base.go	100.00% <100.00%> (ø)
probe/data.go	100.00% <100.00%> (ø)
probe/notification_strategy.go	100.00% <100.00%> (ø)
probe/result.go	100.00% <100.00%> (ø)

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[samanhappy]

LGTM, wonderful work!