added filter on alert by label (#45)

Example run
```
kubectl apply -f https://raw.githubusercontent.com/robusta-dev/kubernetes-demos/main/pending_pods/pending_pod_node_selector.yaml

# wait till alert fires...

poetry run python3 /Users/avirobusta/git/holmesgpt/holmes.py investigate alertmanager --alertmanager-url=http://localhost:9093/ --alert-label-key=pod --alert-label-value=user-profile-import

Configuration file exists at /Users/avirobusta/Library/Preferences/pypoetry, reusing this directory.

Consider moving TOML configuration files to /Users/avirobusta/Library/Application Support/pypoetry, as support for the legacy directory will be removed in an upcoming release.
Analyzing all 1 issues. (Use --alertmanager-alertname to filter.) Press Ctrl+C to stop.
Analyzing issue 1/1: KubePodNotReady...
No runbooks found for this issue. Using default behaviour. (Add runbooks to guide the investigation.)
Running `kubectl describe Pod user-profile-import -n default`                                                                                                                                                                           tools.py:89
────────────────────────────────────────────────────────
AI: Node Affinity Mismatch                                                                                                                                                                                                                             

Resource: Pod user-profile-import                                                                                                                                                                                                                  

Details: Pod user-profile-import is pending because 0/3 nodes match its node affinity/selector criteria.                                                                                                                                           
────────────────────────────────────────────────────────

```

holmes.py

@@ -198,6 +198,9 @@ def alertmanager(
     system_prompt: Optional[str] = typer.Option(
         "builtin://generic_investigation.jinja2", help=system_prompt_help
     ),
+    alertmanager_label: Optional[str] = typer.Option(
+        None, help="For filtering alerts with a specific label must be of format key=value"
+    ),
 ):
     """
     Investigate a Prometheus/Alertmanager alert
@@ -214,6 +217,7 @@ def alertmanager(
         alertmanager_username=alertmanager_username,
         alertmanager_password=alertmanager_password,
         alertmanager_alertname=alertmanager_alertname,
+        alertmanager_label=alertmanager_label,
         slack_token=slack_token,
         slack_channel=slack_channel,
         custom_toolsets=custom_toolsets,

holmes/config.py

@@ -47,6 +47,7 @@ class Config(RobustaBaseConfig):
     alertmanager_username: Optional[str] = None
     alertmanager_password: Optional[str] = None
     alertmanager_alertname: Optional[str] = None
+    alertmanager_label: Optional[str] = None
 
     jira_url: Optional[str] = None
     jira_username: Optional[str] = None
@@ -255,6 +256,7 @@ def create_alertmanager_source(self) -> AlertManagerSource:
             username=self.alertmanager_username,
             password=self.alertmanager_password,
             alertname=self.alertmanager_alertname,
+            label=self.alertmanager_label,
         )
 
     def create_slack_destination(self):

holmes/plugins/sources/prometheus/plugin.py

@@ -25,12 +25,14 @@ def __init__(
         username: Optional[str] = None,
         password: Optional[str] = None,
         alertname: Optional[Pattern] = None,
+        label: Optional[str] = None
     ):
         super().__init__()
         self.url = url
         self.username = username
         self.password = password
         self.alertname = alertname
+        self.label = label
 
     def fetch_issues(self) -> List[Issue]:
         fetch_alerts_url = f"{self.url}/api/v2/alerts"
@@ -55,9 +57,13 @@ def fetch_issues(self) -> List[Issue]:
             a.to_regular_prometheus_alert()
             for a in parse_obj_as(List[PrometheusGettableAlert], data)
         ]
+
+        alerts = self.label_filter_issues(alerts)
+
         if self.alertname is not None:
             alertname_filter = re.compile(self.alertname)
-            alerts = [a for a in alerts if alertname_filter.match(a.unique_id)]
+            alerts = [a for a in alerts if alertname_filter.match(a.unique_id)]   
+
 
         return [
             Issue(
@@ -73,6 +79,17 @@ def fetch_issues(self) -> List[Issue]:
             for alert in alerts
         ]
 
+    def label_filter_issues(self, issues: List[PrometheusAlert]) -> List[PrometheusAlert]: 
+        if not self.label:
+            return issues
+
+        label_parts = self.label.split('=')
+        if len(label_parts) != 2:
+            raise Exception(f"The label {self.label} is of the wrong format use '--alertmanager-label key=value'")
+
+        alert_label_key, alert_label_value = label_parts
+        return [issue for issue in issues if issue.labels.get(alert_label_key, None) == alert_label_value]
+
     @staticmethod
     def __format_issue_metadata(alert: PrometheusAlert) -> Optional[str]:
         if not alert.labels and not alert.annotations: