metrics: add provider rollups

Introduces the following new metrics:

* total_messages_(delivered|fail|transfail)_by_provider
  (labels: provider) - gives aggregates for a given provider/site.
* total_messages_(delivered|fail|transfail)_by_provider_and_source
  (labels: provider, source, pool) - gives aggregates for a given source
  to a specific provider/site.

These queued_count metrics sum the scheduled and ready queue counter
based on the provider, and provider_and_pool:

* queued_count_by_provider
  (labels: provider)
* queued_count_by_provider_and_pool
  (labels: provider, prool)

The provider label in this context is taken from the provider_name that
is set on the associated queue, or if none is set, the site name without
any source or protocol decoration.  This way it is relatively easy to
see at a glance the total queued volume for a given provider/site,
and also to see that broken down by the source *pool*.

And connection counts in a similar veing to the queued_counts:

* connection_count_by_provider
  (labels: provider) - number of connections currently established to
  a provider
* connection_count_by_provider_and_pool
  (labels: provider, pool) - number of connections currently established
  from a pool to a provider.

A new `kcli provider-summary` command shows a provider-centric view of
the queues, using these provider and pool based rollups.

refs: #274

crates/kcli/Cargo.toml

@@ -14,6 +14,7 @@ clap = {version="4.5", features=["derive", "wrap_help"]}
 clap-markdown = "0.1"
 dns-resolver = {path="../dns-resolver"}
 futures = "0.3"
+futures-util = "0.3"
 humantime = "2.1"
 lexicmp = "0.1"
 message = {path="../message", default-features=false}

crates/kcli/src/main.rs

@@ -9,6 +9,7 @@ mod bounce_cancel;
 mod bounce_list;
 mod inspect_message;
 mod logfilter;
+mod provider_summary;
 mod queue_summary;
 mod rebind;
 mod suspend;
@@ -64,6 +65,7 @@ enum SubCommand {
     SuspendReadyQCancel(suspend_ready_q_cancel::SuspendReadyQCancelCommand),
     SetLogFilter(logfilter::SetLogFilterCommand),
     InspectMessage(inspect_message::InspectMessageCommand),
+    ProviderSummary(provider_summary::ProviderSummaryCommand),
     QueueSummary(queue_summary::QueueSummaryCommand),
     TraceSmtpClient(trace_smtp_client::TraceSmtpClientCommand),
     TraceSmtpServer(trace_smtp_server::TraceSmtpServerCommand),
@@ -123,6 +125,7 @@ impl SubCommand {
             Self::SuspendReadyQList(cmd) => cmd.run(endpoint).await,
             Self::SetLogFilter(cmd) => cmd.run(endpoint).await,
             Self::InspectMessage(cmd) => cmd.run(endpoint).await,
+            Self::ProviderSummary(cmd) => cmd.run(endpoint).await,
             Self::QueueSummary(cmd) => cmd.run(endpoint).await,
             Self::TraceSmtpClient(cmd) => cmd.run(endpoint).await,
             Self::TraceSmtpServer(cmd) => cmd.run(endpoint).await,