refactor: use external crate for displaying tracing spans (#4781)

When debugging #4771, I had to copy-paste tracing_timings module to get
a useful profiling output, which isn't ideal. So I improved and
published that code to crates.io as `tracing-span-tree`.

In particular, this new implementation prints the trace in the correct
order, and it also has an `.aggregate(true)` mode for cases where you
run the profiled code in a loop and want a summary.

Example without aggregation:

```
  328.25ms         vm_script_smoke_test
    306.34ms         run_vm
      306.32ms         run_wasmer
        37.16µs          get_key
        301.50ms         compile_and_serialize_wasmer
        4.44ms           run_method
          2.53ms           run_method/instantiate
          128.17µs         run_method/call
          1.74ms           run_method/drop_instance
    4.50ms           run_vm
      4.49ms           run_wasmer
        31.81µs          get_key
        4.25ms           run_method
          2.45ms           run_method/instantiate
          45.46µs          run_method/call
          1.72ms           run_method/drop_instance
    4.43ms           run_vm
      4.42ms           run_wasmer
        22.43µs          get_key
        4.21ms           run_method
          2.41ms           run_method/instantiate
          42.84µs          run_method/call
          1.72ms           run_method/drop_instance
    4.81ms           run_vm
      4.80ms           run_wasmer
        22.09µs          get_key
        4.58ms           run_method
          2.40ms           run_method/instantiate
          419.57µs         run_method/call
          1.73ms           run_method/drop_instance
```

Example with aggregation (unit-less number is the number of aggregated calls):

```
  333.98ms         vm_script_smoke_test
    325.54ms  4      run_vm
      325.48ms  4      run_wasmer
        307.07ms         compile_and_serialize_wasmer
        129.19µs  4      get_key
        17.32ms   4      run_method
          612.46µs  4      run_method/call
          6.63ms    4      run_method/drop_instance
          9.94ms    4      run_method/instantiate
```

Test Plan
---------

This is a debugging utility which is not enabled in production, so I just manually verified that the output makes sense.

runtime/near-vm-runner-standalone/Cargo.toml

@@ -24,8 +24,7 @@ clap = "=3.0.0-beta.2"
 base64 = "0.13"
 strum = "0.20"
 num-rational = "0.3"
-tracing = "0.1"
-tracing-subscriber = "0.2"
+tracing-span-tree = "0.1"
 
 near-vm-logic = { path = "../near-vm-logic", version = "3.0.0", features = ["costs_counting"]}
 near-vm-runner = { path = "../near-vm-runner", version = "3.0.0", features = ["wasmtime_vm", "wasmer1_vm"] }

runtime/near-vm-runner-standalone/src/main.rs

@@ -7,7 +7,6 @@
 //! Optional `--context-file=/tmp/context.json --config-file=/tmp/config.json` could be added
 //! to provide custom context and VM config.
 mod script;
-mod tracing_timings;
 
 use crate::script::Script;
 use clap::Clap;
@@ -121,7 +120,7 @@ fn main() {
     let cli_args = CliArgs::parse();
 
     if cli_args.timings {
-        tracing_timings::enable();
+        tracing_span_tree::span_tree().enable();
     }
 
     let mut script = Script::default();

runtime/near-vm-runner-standalone/src/script.rs

@@ -192,7 +192,7 @@ fn default_vm_context() -> VMContext {
 fn vm_script_smoke_test() {
     use near_vm_logic::ReturnData;
 
-    crate::tracing_timings::enable();
+    tracing_span_tree::span_tree().enable();
 
     let mut script = Script::default();
     script.contract_cache(true);
@@ -242,7 +242,7 @@ fn profile_data_is_per_outcome() {
 fn test_evm_slow_deserialize_repro() {
     fn evm_slow_deserialize_repro(vm_kind: VMKind) {
         println!("evm_slow_deserialize_repro of {:?}", &vm_kind);
-        crate::tracing_timings::enable();
+        tracing_span_tree::span_tree().enable();
 
         let mut script = Script::default();
         script.vm_kind(vm_kind);

runtime/near-vm-runner/README.md

@@ -66,10 +66,10 @@ tracing_subscriber::fmt::Subscriber::builder()
 code_to_profile_here();
 ```
 
-Alternatively, in the `standalone` crate we have a more compact hierarchical subscriber
+Alternatively, there's an alternative hierarchical profiler
 
 ```rust
-crate::tracing_timings::enable();
+tracing_span_tree::span_tree().enable();;
 
 code_to_profile_here();
 ```