Grafana needs to know scrape_interval for $__rate_interval

Grafana assumes the scrape interval as 15s by default but COS stack
assumes 1m. Without telling Grafana that COS Prometheus uses 1m as the
global scrape interval, many graphs will break since $__rate_interval
won't be calculated properly.

Hardcoding 1m itself is in question, but this patch fixes many graphs
using $__rate_interval without touching the fundamental pieces.

ref:
https://grafana.com/docs/grafana/latest/datasources/prometheus/configure-prometheus-data-source/#interval-behavior
https://grafana.com/blog/2020/09/28/new-in-grafana-7.2-__rate_interval-for-prometheus-rate-queries-that-just-work/
#544

Closes: #543

src/charm.py

@@ -63,6 +63,7 @@
 
 PROMETHEUS_DIR = "/etc/prometheus"
 PROMETHEUS_CONFIG = f"{PROMETHEUS_DIR}/prometheus.yml"
+PROMETHEUS_GLOBAL_SCRAPE_INTERVAL = "1m"
 RULES_DIR = f"{PROMETHEUS_DIR}/rules"
 CONFIG_HASH_PATH = f"{PROMETHEUS_DIR}/config.sha256"
 ALERTS_HASH_PATH = f"{PROMETHEUS_DIR}/alerts.sha256"
@@ -171,6 +172,7 @@ def __init__(self, *args):
             source_type="prometheus",
             source_url=self.internal_url,  # https://github.com/canonical/operator/issues/970
             refresh_event=self.cert_handler.on.cert_changed,
+            extra_fields={"timeInterval": PROMETHEUS_GLOBAL_SCRAPE_INTERVAL},
         )
 
         self.catalogue = CatalogueConsumer(charm=self, item=self._catalogue_item)
@@ -827,7 +829,10 @@ def _prometheus_global_config(self) -> dict:
             a dictionary consisting of global configuration for Prometheus.
         """
         config = self.model.config
-        global_config = {"scrape_interval": "1m", "scrape_timeout": "10s"}
+        global_config = {
+            "scrape_interval": PROMETHEUS_GLOBAL_SCRAPE_INTERVAL,
+            "scrape_timeout": "10s",
+        }
 
         if config.get("evaluation_interval") and self._is_valid_timespec(
             config["evaluation_interval"]