Replace consts with static readonly

[AndreyTretyak]

Using const results in a new allocation each time it's used. The most common exception would be health check markers we have that used each time we mark activity as healthcheck or check it.

This PR replaces const with static readonly to avoid additional allocations.

[syphe]

Another reason this is a good idea, is that const is determined at compile time when being referenced elsewhere, where as 'static readonly' will cause the value to be referenced at runtime instead.

[artempushkin]

Let's look closer at those extra allocations, I don't see where they would be coming from.

[K-Cully]

This class is already static so I expect these shouldn't be changed.

[K-Cully]

class is already static

[K-Cully]

class is already static

[K-Cully]

class is static

[K-Cully]

Let's look closer at those extra allocations, I don't see where they would be coming from.

I take it that the issue is that as transient instances get created and destroyed as part of their DI lifecycle, the const values are being repeatedly allocated.
However, this change appears to be over scoped, since that would not affect static classes.

[artempushkin]

Let's look closer at those extra allocations, I don't see where they would be coming from.

I take it that the issue is that as transient instances get created and destroyed as part of their DI lifecycle, the const values are being repeatedly allocated. However, this change appears to be over scoped, since that would not affect static classes.

They shouldn't be, CLI ensures that each string constant exists as a singular object (though you can trick it if you try hard). I was talking to Andrey separately, profiler detects multiple allocations of some strings but I'm sure this PR won't fix it.

[artempushkin]

I'll block this to prevent accidental merging.