stats: the writers of the document

How many writers (Automerge actors) the document's changes have matters: a writer is a
node's run (each start, rejoin or drop makes one, a start keeping the one of before with
`--keep-actor` after a clean stop), and every transfer of the whole history carries a
signature for each. `/admin/stats` now says it (`document.writers`, and
`document_writers` for Prometheus), counted from the changes' metadata alongside the
saved size's measurement in the background (at least a minute apart), as that takes
time proportional to the document too; an export, which measures the size alone, keeps
the count of before. The dashboard's figures show it with the rest.

1 files changed, 8 insertions, 0 deletions

diff --git a/src/api/stats/prometheus.rs b/src/api/stats/prometheus.rs
index 0d79d2c..ca3f1af 100644
--- a/src/api/stats/prometheus.rs
+++ b/src/api/stats/prometheus.rs
@@ -460,6 +460,14 @@ pub(super) fn render(stats: &StatsResponse, list: &PeerListResponse) -> String {
         "Most heads after applying a sync message, since start.",
         document.max_heads as f64,
     );
+    if let Some(writers) = document.writers {
+        out.single(
+            "document_writers",
+            "gauge",
+            "Writers (Automerge actors, a node's run each) with changes in the document; counted in the background.",
+            writers as f64,
+        );
+    }
     if let Some(bytes) = document.saved_bytes {
         out.single(
             "document_saved_bytes",