feat: add load_based selector in meta

[fengys1996]

I hereby agree to the terms of the GreptimeDB CLA

What's changed and what's your intention?

This pr mainly change:

1. add load_based selector. (Now it is mainly based on the number of regions of the datanode)
2. add a attrs filed in Context, which used to pass some paramters to Selector.
3. fix wrong error display.

How to use load_based or lease_based selector? Default lease_based.

cargo run -- metasrv start --selector LoadBased
cargo run -- metasrv start --selector LeaseBased 

Checklist

• I have written the necessary rustdoc comments.
• I have added the necessary unit tests and integration tests.

Refer to a related PR or issue link (optional)

[codecov]

Codecov Report

Merging #874 (53ceb62) into develop (2664436) will decrease coverage by 0.09%.
The diff coverage is 85.15%.

@@             Coverage Diff             @@
##           develop     #874      +/-   ##
===========================================
- Coverage    86.28%   86.19%   -0.10%     
===========================================
  Files          425      427       +2     
  Lines        55858    57260    +1402     
===========================================
+ Hits         48199    49353    +1154     
- Misses        7659     7907     +248     

Flag	Coverage Δ
rust	86.19% <85.15%> (-0.10%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
src/cmd/src/error.rs	91.78% <0.00%> (-1.28%)	⬇️
src/meta-srv/src/bootstrap.rs	0.00% <0.00%> (ø)
src/meta-srv/src/error.rs	95.32% <ø> (ø)
src/meta-srv/src/handler.rs	100.00% <ø> (ø)
src/meta-srv/src/selector/load_based.rs	77.69% <77.69%> (ø)
src/meta-srv/src/selector.rs	85.29% <85.29%> (ø)
src/cmd/src/metasrv.rs	70.51% <90.90%> (+3.34%)	⬆️
src/meta-srv/src/handler/node_stat.rs	77.94% <95.23%> (-0.19%)	⬇️
src/meta-srv/src/keys.rs	98.12% <97.82%> (+0.42%)	⬆️
src/meta-srv/src/handler/persist_stats_handler.rs	97.53% <100.00%> (+0.09%)	⬆️
... and 51 more

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[sunng87]

1. it looks like this patch does not contain usage of LoadBasedSelector so I guess this is part 1 with some more patches following up/
2. I suggest to avoid using AnyMap when it's possible because of it's opaque at compile time. For context, we can include as much information as needed.