Expose epoch schedule metrics in Prometheus

With the epoch start slots and the number of slots in the epoch (and the
current slot, which we already had), we can infer/estimate:

 * Epoch progress percentage
 * Slots left until the next epoch
 * Time left until the next epoch (from slot height increase)

These are useful metrics to have about the network.

prometheus/src/bank_metrics.rs

@@ -3,6 +3,8 @@ use crate::{
     utils::{write_metric, Metric, MetricFamily},
 };
 use std::io;
+use solana_sdk::sysvar;
+use solana_sdk::sysvar::epoch_schedule::EpochSchedule;
 
 pub fn write_bank_metrics<W: io::Write>(
     banks_with_commitments: &BanksWithCommitments,
@@ -28,6 +30,46 @@ pub fn write_bank_metrics<W: io::Write>(
                 .for_each_commitment(|bank| Some(Metric::new(bank.clock().epoch))),
         },
     )?;
+    write_metric(
+        out,
+        &MetricFamily {
+            name: "solana_block_epoch_start_slot",
+            help: "The first slot in the current epoch",
+            type_: "gauge",
+            metrics: banks_with_commitments
+                .for_each_commitment(|bank| {
+                    // Note, the bank actually has a field that holds the EpochSchedule,
+                    // but it is not public, so we can't easily access it  here. We could
+                    // make it public, but to make our patches less invasive, load the
+                    // epoch schedule from the sysvar instead. It should always exist.
+                    let epoch_schedule: EpochSchedule = bank
+                        .get_account(&sysvar::epoch_schedule::id())?
+                        .deserialize_data().ok()?;
+                    let clock = bank.clock();
+                    Some(Metric::new(epoch_schedule.get_first_slot_in_epoch(clock.epoch)))
+                }),
+        },
+    )?;
+    write_metric(
+        out,
+        &MetricFamily {
+            name: "solana_block_epoch_slots_total",
+            help: "The duration of the current epoch, in slots.",
+            type_: "gauge",
+            metrics: banks_with_commitments
+                .for_each_commitment(|bank| {
+                    // Note, the bank actually has a field that holds the EpochSchedule,
+                    // but it is not public, so we can't easily access it  here. We could
+                    // make it public, but to make our patches less invasive, load the
+                    // epoch schedule from the sysvar instead. It should always exist.
+                    let epoch_schedule: EpochSchedule = bank
+                        .get_account(&sysvar::epoch_schedule::id())?
+                        .deserialize_data().ok()?;
+                    let clock = bank.clock();
+                    Some(Metric::new(epoch_schedule.get_slots_in_epoch(clock.epoch)))
+                }),
+        },
+    )?;
     write_metric(
         out,
         &MetricFamily {