Add metrics to the index

[tobim]

📝 Checklist

• A metrics heartbeat is added back to the index.
• All user-facing changes have changelog entries.
• The changes are reflected on docs.tenzir.com/vast, if necessary.
• The PR description contains instructions for the reviewer, if necessary.

🎯 Review Instructions

Commits 2 and 3 are only moving code around and don't contain any logical changes.

[dominiklohmann]

The metrics work as expected. Here's a diff that makes the unit tests compile:

diff --git a/libvast/test/system/eraser.cpp b/libvast/test/system/eraser.cpp
index b60dfb991..f0b478c1e 100644
--- a/libvast/test/system/eraser.cpp
+++ b/libvast/test/system/eraser.cpp
@@ -70,6 +70,9 @@ mock_index(system::index_actor::stateful_pointer<mock_index_state> self) {
     [=](system::accountant_actor&) {
       FAIL("no mock implementation available");
     },
+    [=](atom::telemetry) {
+      FAIL("no mock implementation available");
+    },
     [=](atom::status, system::status_verbosity) -> record {
       FAIL("no mock implementation available");
     },
diff --git a/libvast/test/system/query_processor.cpp b/libvast/test/system/query_processor.cpp
index 5712a0017..d72406d51 100644
--- a/libvast/test/system/query_processor.cpp
+++ b/libvast/test/system/query_processor.cpp
@@ -49,6 +49,9 @@ mock_index(system::index_actor::stateful_pointer<mock_index_state> self) {
     [=](system::accountant_actor) {
       FAIL("no mock implementation available");
     },
+    [=](atom::telemetry) {
+      FAIL("no mock implementation available");
+    },
     [=](atom::status, system::status_verbosity) -> record {
       FAIL("no mock implementation available");
     },