chore: add total-transactions metric

[mattsse]

while this should always be the sum of all the other size fields, it's good to have so we can check that this is actually true.

@Rjected do we have these metrics on the dashboard?

[codecov]

Codecov Report

Merging #3500 (a5547a2) into main (b225b47) will increase coverage by 0.02%.
The diff coverage is 0.00%.

Impacted Files	Coverage Δ
crates/transaction-pool/src/metrics.rs	100.00% <ø> (ø)
crates/transaction-pool/src/pool/txpool.rs	60.41% <0.00%> (-0.13%)	⬇️
crates/transaction-pool/src/traits.rs	5.26% <ø> (ø)

... and 11 files with indirect coverage changes

Flag	Coverage Δ
integration-tests	16.26% <0.00%> (+0.01%)	⬆️
unit-tests	64.24% <0.00%> (+0.02%)	⬆️

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

Components	Coverage Δ
reth binary	23.05% <ø> (ø)
blockchain tree	81.69% <ø> (ø)
pipeline	87.88% <ø> (ø)
storage (db)	73.93% <ø> (ø)
trie	95.64% <ø> (ø)
txpool	51.79% <0.00%> (+0.61%)	⬆️
networking	77.90% <ø> (ø)
rpc	58.13% <ø> (-0.01%)	⬇️
consensus	62.78% <ø> (ø)
revm	35.07% <ø> (ø)
payload builder	6.83% <ø> (ø)
primitives	88.70% <ø> (+0.02%)	⬆️