Remove fallback support from eth1 service (#3594)

## Issue Addressed

N/A

## Proposed Changes

With https://github.com/sigp/lighthouse/pull/3214 we made it such that you can either have 1 auth endpoint or multiple non auth endpoints. Now that we are post merge on all networks (testnets and mainnet), we cannot progress a chain without a dedicated auth execution layer connection so there is no point in having a non-auth eth1-endpoint for syncing deposit cache. 

This code removes all fallback related code in the eth1 service. We still keep the single non-auth endpoint since it's useful for testing.

## Additional Info

This removes all eth1 fallback related metrics that were relevant for the monitoring service, so we might need to change the api upstream.

common/fallback/Cargo.toml

@@ -1,10 +0,0 @@
-[package]
-name = "fallback"
-version = "0.1.0"
-authors = ["blacktemplar <blacktemplar@a1.net>"]
-edition = "2021"
-
-# See more keys and their definitions at https://doc.rust-lang.org/cargo/reference/manifest.html
-
-[dependencies]
-itertools = "0.10.0"

common/fallback/src/lib.rs

@@ -1,63 +0,0 @@
-use itertools::{join, zip};
-use std::fmt::{Debug, Display};
-use std::future::Future;
-
-#[derive(Clone)]
-pub struct Fallback<T> {
-    pub servers: Vec<T>,
-}
-
-#[derive(Debug, PartialEq)]
-pub enum FallbackError<E> {
-    AllErrored(Vec<E>),
-}
-
-impl<T> Fallback<T> {
-    pub fn new(servers: Vec<T>) -> Self {
-        Self { servers }
-    }
-
-    /// Return the first successful result along with number of previous errors encountered
-    /// or all the errors encountered if every server fails.
-    pub async fn first_success<'a, F, O, E, R>(
-        &'a self,
-        func: F,
-    ) -> Result<(O, usize), FallbackError<E>>
-    where
-        F: Fn(&'a T) -> R,
-        R: Future<Output = Result<O, E>>,
-    {
-        let mut errors = vec![];
-        for server in &self.servers {
-            match func(server).await {
-                Ok(val) => return Ok((val, errors.len())),
-                Err(e) => errors.push(e),
-            }
-        }
-        Err(FallbackError::AllErrored(errors))
-    }
-
-    pub fn map_format_error<'a, E, F, S>(&'a self, f: F, error: &FallbackError<E>) -> String
-    where
-        F: FnMut(&'a T) -> &'a S,
-        S: Display + 'a,
-        E: Debug,
-    {
-        match error {
-            FallbackError::AllErrored(v) => format!(
-                "All fallbacks errored: {}",
-                join(
-                    zip(self.servers.iter().map(f), v.iter())
-                        .map(|(server, error)| format!("{} => {:?}", server, error)),
-                    ", "
-                )
-            ),
-        }
-    }
-}
-
-impl<T: Display> Fallback<T> {
-    pub fn format_error<E: Debug>(&self, error: &FallbackError<E>) -> String {
-        self.map_format_error(|s| s, error)
-    }
-}

common/monitoring_api/src/gather.rs

@@ -43,6 +43,16 @@ impl JsonMetric {
             }
         }
     }
+
+    /// Return a default json value given given the metric type.
+    fn get_typed_value_default(&self) -> serde_json::Value {
+        match self.ty {
+            JsonType::Integer => json!(0),
+            JsonType::Boolean => {
+                json!(false)
+            }
+        }
+    }
 }
 
 /// The required metrics for the beacon and validator processes.
@@ -155,6 +165,16 @@ pub fn gather_metrics(metrics_map: &HashMap<String, JsonMetric>) -> Option<serde
             let _ = res.insert(metric.json_output_key.to_string(), value);
         };
     }
+    // Insert default metrics for all monitoring service metrics that do not
+    // exist as lighthouse metrics.
+    for json_metric in metrics_map.values() {
+        if !res.contains_key(json_metric.json_output_key) {
+            let _ = res.insert(
+                json_metric.json_output_key.to_string(),
+                json_metric.get_typed_value_default(),
+            );
+        }
+    }
     Some(serde_json::Value::Object(res))
 }