adding max_token_capacity metric

[Edwinhr716]

What does this PR do?

Emit max_batch_total_tokens as max_token_capacity as part of metrics standardization

Fixes # (issue)

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  This has been discussed in [Feature]: Additional metrics to enable better autoscaling / load balancing of TGI servers in Kubernetes #1977, as part of metric standardization
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[achandrasekar]

@Narsil can you take a look at this one?

[achandrasekar]

I wanted to ping on this. There is already a tgi_batch_current_max_tokens metric which reports the current batch size in terms of tokens. Adding this max_token_capacity alongside would allow us to see how far off from the max capacity we are at and helps us with autoscaling proactively where we can say we are at 75% capacity and trigger a scale up.