Implement skip_datasets for monitor rules

Every item can be configured to skip some datasets from the check, like:

```yaml
- prefix: "zrepl_monthly_"
  skip_datasets:
    - pattern: "zdisk/video"
  warning: 13
  critical: 14
```

In this example it checks number of snapshots with prefix `zrepl_monthly_` for
every dataset, configured in `datasets`, except `zdisk/video`. `skip_datasets`
has the same syntax, like `datasets`.

README.md

@@ -313,7 +313,7 @@ pkg install zrepl-dsh2dsh
     is ok, latest or oldest snapshots are not too old. See
     [#765](https://github.com/zrepl/zrepl/pull/765). Configuration example:
 
-    ``` yaml
+    ```yaml
     monitor:
       count:
         - prefix: "zrepl_frequently_"
@@ -353,9 +353,23 @@ pkg install zrepl-dsh2dsh
           critical: "168h"      # 7d
     ```
 
-    Example of a daily script:
+    Every item can be configured to skip some datasets from the check, like:
+
+    ```yaml
+    - prefix: "zrepl_monthly_"
+      skip_datasets:
+        - pattern: "zdisk/video"
+      warning: 13
+      critical: 14
+    ```
+
+    In this example it checks number of snapshots with prefix `zrepl_monthly_`
+    for every dataset, configured in `datasets`, except `zdisk/video`.
+    `skip_datasets` has the same syntax, like `datasets`.
+
+    An example of a daily script:
 
-    ``` shell
+    ```shell
     echo
     echo "zrepl status:"
     zrepl monitor alive

internal/client/monitor/rules.go

@@ -0,0 +1,91 @@
+package monitor
+
+import (
+	"github.com/dsh2dsh/zrepl/internal/config"
+	"github.com/dsh2dsh/zrepl/internal/daemon/filters"
+	"github.com/dsh2dsh/zrepl/internal/zfs"
+)
+
+func CountRulesFromConfig(in []config.MonitorCount) ([]*CountRule, error) {
+	rules := make([]*CountRule, len(in))
+	for i := range in {
+		r, err := NewCountRule(&in[i])
+		if err != nil {
+			return nil, err
+		}
+		rules[i] = r
+	}
+	return rules, nil
+}
+
+func NewCountRule(m *config.MonitorCount) (*CountRule, error) {
+	r := &CountRule{MonitorCount: m}
+	return r.init()
+}
+
+type CountRule struct {
+	*config.MonitorCount
+
+	skip *filters.DatasetFilter
+}
+
+func (self *CountRule) init() (*CountRule, error) {
+	if length := len(self.SkipDatasets); length != 0 {
+		skip := filters.New(length)
+		if err := skip.AddList(self.SkipDatasets); err != nil {
+			return nil, err
+		}
+		self.skip = skip
+	}
+	return self, nil
+}
+
+func (self *CountRule) Skip(path *zfs.DatasetPath) (bool, error) {
+	if self.skip != nil {
+		return self.skip.Filter(path)
+	}
+	return false, nil
+}
+
+// --------------------------------------------------
+
+func AgeRulesFromConfig(in []config.MonitorCreation) ([]*AgeRule, error) {
+	rules := make([]*AgeRule, len(in))
+	for i := range in {
+		r, err := NewAgeRule(&in[i])
+		if err != nil {
+			return nil, err
+		}
+		rules[i] = r
+	}
+	return rules, nil
+}
+
+func NewAgeRule(m *config.MonitorCreation) (*AgeRule, error) {
+	r := &AgeRule{MonitorCreation: m}
+	return r.init()
+}
+
+type AgeRule struct {
+	*config.MonitorCreation
+
+	skip *filters.DatasetFilter
+}
+
+func (self *AgeRule) init() (*AgeRule, error) {
+	if length := len(self.SkipDatasets); length != 0 {
+		skip := filters.New(length)
+		if err := skip.AddList(self.SkipDatasets); err != nil {
+			return nil, err
+		}
+		self.skip = skip
+	}
+	return self, nil
+}
+
+func (self *AgeRule) Skip(path *zfs.DatasetPath) (bool, error) {
+	if self.skip != nil {
+		return self.skip.Filter(path)
+	}
+	return false, nil
+}